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chipchip
March 6, 2014, 02:37 AM
Why do some rifle manufactures free float thier rifle barrels while others use a pressure point. Is one better than another.

iraiam
March 6, 2014, 06:32 AM
A pressure point on a barrel stiffens it, I always assumed that most rifles that have a stock with a pressure point on the barrel was less expensive and faster to manufacture.

I base this assumption on the rifles that I have built with free floated barrels, pillars and bedding takes more labor and material.

IMO, a free floated barrel with proper action bedding and/or pillars, is not necessarily more accurate, but rather more stable over time, and through changing environmental conditions.

One of my most accurate rifles is a Remington 700 BDL in 270 win, this is a wood stocked rifle with a pressure point on the barrel, it's so accurate, I cannot justify changing it.

Doyle
March 6, 2014, 08:06 AM
I've always thought that a pressure point had the most positive influence on a thin barrel. As barrel thickness increases, the benefit of a pressure point goes away.

Bart B.
March 6, 2014, 09:01 AM
Do those pressure points always have the same amount of force and in the same direction on the barrel?

If so, I think the stock's material from the grip forward would have to be so stiff it would never bend from any outside influence. What material is that stiff?

Decades ago, Remington tried that on their 40X target rifles with two screws 90 degrees apart under the barrel at the fore end's tip. Each one was supposed to be set to some pressure amount against the barrel. Competitive shooters quickly learned that was a joke because the rifle's fore end bent enough in different positions it was fired from that zeros changed and accuracy suffered. With those two screws backed out to allow clearance, good accuracy and consistant zeros returned.

If true, why do all the best performing competition rifles have totally free floating barrels?

Jimro
March 6, 2014, 09:07 AM
If true, why do all the best performing competition rifles have totally free floating barrels?

Because the best performing competition rifles are designed and built to compete.

If you have a hunting rifle that is finicky about accuracy, a pressure bed or full length bed can sometimes tighten groups to get you acceptable accuracy without the cost of a rebarrel job. I sort of view it as an option of last resort, especially a full length bed job on a barrel which will generally really tighten groups for the first three shots, but you shouldn't be taking more than three with a big game rifle anyways right?

Jimro

Doyle
March 6, 2014, 09:20 AM
Here is what I do when I pick up a 2nd hand hunting rifle (with a wood stock) that needs tuning. I sand the barrel channel to float the barrel down to the thick bell just in front of the recoil lugs and re-seal the wood. Then, I shoot a nice slow 3-shot group. I then use a piece of folded card stock to create a temporary pressure point under the barrel and shoot another slow 3-shot group. If the groups improve, then I've concluded that this particular barrel will benefit from a pressure point. If the group stays the same or gets worse, then that rifle gets no pressure point.

My method may not be the best but it is simple and works well enough for me.

Art Eatman
March 6, 2014, 09:43 AM
I'm with Doyle on this, although I've always found that leaving the light pressure is a good thing. My uncle showed me that trick almost fifty years ago. His theory was that it dampens the vibrations and assists uniformity from shot to shot.

I open up the barrel channel in the forearm for clearance, but leave it pretty close at the front inch of the forearm.

I cut a 3/4" strip of kitchen wax paper (won't absorb moisture) and fold it back and forth until it takes about a five-pound pull to separate the barrel and forearm to allow insertion. Shooting melts the wax enough so that it sticks in place. (Maybe the melting reduces the force a little; I dunno.)

The main thing is that it has always been helpful.

Bart B.
March 6, 2014, 09:53 AM
There's no way a pressure on a barrel will change its metalurgy nor anything else to make it stiffer. It only bends the barrel away from its natural position. Its axis will change the barrel's whip axis and its force will change the barrel's whip ampitude. And it often adds an extra node in the barrel's whip/vibration patterns.

There's no way a pressure point's force and direction on a barrel will remain constant in all parameters the rifle's fired in.

Shooting a rifle whose fore end's resting atop something on a bench as it's held against ones shoulder puts different pressures from that contact point on the barrel in other shooting postions. It varies with the front to back position the rest is under the fore end as well as the down force on the shooter's face on the stock comb/cheekpiece.

That's easy to see by measuring how much the barrel bends (from its natural line of fire axis) with a pressure point on it from any place in the stock's fore end; especially at its tip. Measure yours and you'll understand.

I've won all my bets on this issue.

jmr40
March 6, 2014, 10:30 AM
Talking about 2 different things here. Accuracy and consistency are not the same things. MOST rifles shoot more accurately free floated. SOME rifles, especially those with barrels on the thin side are more accurate with some pressure on them. But that doesn't mean they will always shoot those accurate groups to the same point of impact. As environmental conditions change the amount of pressure changes, and that changes point of impact.

You sometimes have to make a choice. You can have a hunting rifle with a pressure point that shoots 1 MOA, but as the seasons change the POI may change by several inches. Or you can free float the barrel and have a 1.5 MOA rifle that consistently puts the bullets into the same spot on the target.

This isn't always the case. But I've seen it happen. I think you just have to experiment with individual rifles, but I prefer to try free floating first. You can always add a pressure point later if not satisfied.

Bart B.
March 6, 2014, 10:56 AM
jmr40, point of impact, or zero, changes with shooting position and stance if the barrel has a pressure point on it from the fore end. Sight settings are not the same from benchrest, to standing to prone or resting the fore end atop a tree limb afield. The stiffer the barrel is, the less it bends from that external force against it.

That's easy to measure and see without firing a shot. And how much the barrel changes its line of fire axis can easily be seen to 1/20th MOA or more resolution. Sometimes, it's as much a 2 MOA.

People that have measured this understand.

A 30 caliber 30 inch long 5.1 pound Palma barrel or a 26 inch 4.4 pound heavy sporter barrel is not as stiff as a 22 inch long 2.1 pound featherweight 30 caliber barrel. That Palma and heavy sporter barrels resonant frequency is about 39 Hz while the featherweight one's 64 Hz. Stiffer barrels resonate at higher frequencies than those not as stiff. Nobody I know of pressure beds their Palma rifle barrels.

baddarryl
March 6, 2014, 11:06 AM
BartB- Stupid question here, but how do you measure that? With a straight edge or something?

Bart B.
March 6, 2014, 11:09 AM
baddarryl, are you asking how I measure bending barrels? If so I'll check with my secruity group manager and find out if I can release that top secret way to measure how much a barrel bends from external forces. All I can say now that it does involve something very, very straight. Your question revealed your good intellignece based desire to learn something; you're not stupid at all.

Meanwhile, I'll answer chipchip's query:Why do some rifle manufactures free float thier rifle barrels while others use a pressure point?I would ask them to get their reasoning.

It's my opinion that pressure points are put in to make a poorly bedded receiver (or complete barreled action) enable a little better accuracy. Those making rifles free floating barrels probably know that's a better way and fit their receivers much better to the stock. No pressure on the barrel from any external source is a constant; any pressure is a variable. What do you want your rifle to perform at its best with in all shooting environments?

Bart B.
March 6, 2014, 02:08 PM
baddarryl, Security Group's cleared you to get my top secret stuff. PM's been sent to you.

Jimro
March 6, 2014, 02:27 PM
Bart B.

Everything you write is true, but I think it offers an incomplete picture about what makes an accurate barrel.

There's no way a pressure on a barrel will change its metalurgy nor anything else to make it stiffer. It only bends the barrel away from its natural position. Its axis will change the barrel's whip axis and its force will change the barrel's whip ampitude. And it often adds an extra node in the barrel's whip/vibration patterns.

There's no way a pressure point's force and direction on a barrel will remain constant in all parameters the rifle's fired in.

This is true, but it can change the frequencies that the barrel is vibrating at the same way your finger changes the pitch on an instrument string.

A 30 caliber 30 inch long 5.1 pound Palma barrel or a 26 inch 4.4 pound heavy sporter barrel is not as stiff as a 22 inch long 2.1 pound featherweight 30 caliber barrel. That Palma and heavy sporter barrels resonant frequency is about 39 Hz while the featherweight one's 64 Hz. Stiffer barrels resonate at higher frequencies than those not as stiff. Nobody I know of pressure beds their Palma rifle barrels.

Frequency is essentially meaningless without also talking about amplitude. The 2.1 lb sporter barrel will have LARGER peaks and valleys than the less stiff 5.1 Palma barrel. There is less mass (under 1 Kg) in the lightweight sporter than in the 5.1 (over 2 Kg) Palma barrel. If you have the same load shooting through both barrels, call it a 308 Win in this case, you'll see that the barrel deflections will be greater with the stiffer lightweight option because there is less mass to resist movment. Force equals mass times acceleration. When the acceleration is the same (the 308 Win) and mass is reduced, then the acceleration will increase to compensate for conservation of energy.

What pressure bedding does is "dampen" the barrel the same way a finger changes the pitch on an instrument string. A barrel tuner works the same way, moving mass forward and backward until the right resonance at the muzzle is achieved for maximum accuracy.

Generally when a barrel "won't shoot" it means that the loads running through that barrel are leaving at a "scatter node" in terms of barrel timing. The solution is to either try different loads, or to change the resonance of the barrel to have the bullets leave at something other than a scatter node.

So that is why pressure bedding works, and why it isn't used in top performing competition rifles.

Jimro

Slamfire
March 6, 2014, 03:38 PM
I have never owned a rifle that shot better with a pressure bedded barrel and the same experience is true for the Gun Club President. He also beds and experiments with his rifles.

I went through bedding two M700 Remington's. One in 6.5 Swede and another in 30-06.

I conducted load development for M700 classic in 6.5 Swede and found the thing was not necessarily a tack driver. At 100 yards it did shoot under 2”, which I consider perfectly acceptable for deer hunting. If the rifle shoots 2 MOA, then it will hit within four inches at 200 yards, six inches at 300 yards. That is plenty good, considering that I don’t hold much better with a lightweight rifle off the bench.

The 6.5 Swede action was in a wooden stock. For this rifle, and someone else confirmed their rifle was similar, Remington created a raised area in the barrel channel which created a pressure point. I like free floated barrels. When a barrel heats up it will expand. If there is a pressure point, or a bearing point on the barrel, as the barrel expands, the pressure against the barrel changes. This will cause a change in a point of impact.

So with stock channel tools, I scraped the barrel channel, removing the pressure point, and created a clearance so the barrel no longer touched the left side of the barrel channel. I suspect the left side of the barrel touching the stock created a lot of side to side movements. But not all. If the recoil lug is free to slide around in the stock, the action will shift during recoil.

I “pillar” bedded creating columns of Bisonite, and then I routed a humongous amount of wood forward of the magazine recess, and filled that with Bisonite. The final bedding looks awful, with voids, and it is not completely filled out around the recoil lug recess. But I was tired and grumpy and wanted to shoot my rifle, so I put it back together and took it to the range.

Anyway, just bedding and free floating the barrel of this rifle changed its group size considerably. These lightweight rifles are hard to shoot, they are twitchy, they kick hard, and they are very sensitive to stock weld and shooting position. Still, this rifle might shoot under 1 MOA, which is excellent for a deer rifle.


I have couple of "before glassbedding" targets. Everything is at 100 yards. If you notice the wide horizontal dispersion with 140 SMK’s. Sierra match bullets are in a word, superb. In a match barrel they will shoot bug hole groups. This side to side movement indicated to me that something was wrong with the bedding. The action, or the barrel was moving left and right in the stock.


http://img.photobucket.com/albums/v479/SlamFire/M700%20Remingtons/ReducedBGlass140SMK391AA2700.jpg (http://smg.photobucket.com/user/SlamFire/media/M700%20Remingtons/ReducedBGlass140SMK391AA2700.jpg.html)


In my opinion it shot much better. These targets were fired fast, about five shots under a minute, maybe two. I racked the bolt and shot if the crosshairs looked good. The barrel was hot enough to be uncomfortable to touch.

http://img.photobucket.com/albums/v479/SlamFire/M700%20Remingtons/Reduced140Hornady43AA4350t2.jpg (http://smg.photobucket.com/user/SlamFire/media/M700%20Remingtons/Reduced140Hornady43AA4350t2.jpg.html)


This improvement so encouraged me, that I bought a new stock for my other M700. It is a 30-06 in a Remington "tupperware" stock. I am going to put it in a laminated stock, and I am going to pillar that. I hope that I can get it to shoot under MOA.The 30-06 was removed from its factory plastic stock and bedded in a laminated stock. I believe laminated stocks are very stiff.

http://img.photobucket.com/albums/v479/SlamFire/M700%20Remingtons/FulllengthDSCN8756.jpg

The 30-06, I did not shoot enough pre bedding targets, but this is one.

http://img.photobucket.com/albums/v479/SlamFire/M700%20Remingtons/ReducedRem700E6251527174FMJBT47.jpg

This is a post bedding target.

http://img.photobucket.com/albums/v479/SlamFire/M700%20Remingtons/175SMK55H4350Rem700.jpg


This is not quite an apples to apples comparison, the first target was fired using 174 FMJ and surplus 4895, and the second was 175 SMK’s and 4350. And the groups are not tiny clusters, but I do believe the post bedded rifle is more accurate. I am quite impressed that the factory hunting barrels on both rifles are capable of MOA grouping. It used to be that only an exceptional factory barrel would shoot MOA.

Art Eatman
March 6, 2014, 05:13 PM
One thing that I make a point of when sighting in at my benchrest is to make sure that the front sandbag is where I put my hand when in the field. I've also compared POI with my hand on top of that front sandbag. Same POI, either way.

All I know is that my deal improved the groups from before I tried it, and I don't argue with 43 years of sub-MOA from my two pet sporters.

Doyle
March 6, 2014, 05:35 PM
Art, if you are willing to try an experiment next time you go to put in your permanent pressure point try a drop of ordinary caulk. Put it on and let it dry for about a full day - maybe longer. You want it hard but still somewhat pliable. Then, screw the action onto the stock.

I've yet to try it myself but an old shooter at the range told me that's how he's been doing it. It's certainly cheap and it can be easily removed if you want. Plus, unlike paper - it isn't going to slide around on you.

Art Eatman
March 6, 2014, 05:52 PM
As I said, heat makes the wax of the wax paper stick to the barrel. It doesn't move, once in place.

Thanks for the tip, but I'm pushing 80 and sorta out of the new-rifle business. :)

Maybe another way to see my deal is to think of just a light pressure to dampen harmonics. Functions much like shock absorbers on a car. Negative feedback.

I did the same thing with a few of my gun-show trade-ins, tweaking them some before resale. Similar results for improved groups.

Bart B.
March 6, 2014, 07:49 PM
Jimro, comparing a rifle barrel to a guitar string on its instrument is not realistic. One's fixed at one end and not under any axial force then shocked into S-shaped vibrations by a quick upward swing at its back end and whips at 64 Hz. The other fixed at both ends with several dozen pounds of force pulling on it and is shocked by bending its middle out then letting it vibrate in smaller S shapes at a few hundred Hz or more. Vibration frequencies and shapes are significantly different between them.

I'll put a guitar spring in the same environment as the barrel. A guitar's steel B string about 2 feet long and .013" diameter has a weight of 1/70th ounce and a fundamental frequency of 0.6291 Hz. If shocked the same as the barrel, it will whip one cycle every 1.59 seconds. I've no idea what it would do if you put your finger on it while it was wiggling.

Art Eatman
March 6, 2014, 09:09 PM
Don't get me wrong. I'm not at all saying anything against a free-floated barrel. About 15 years back I had a Bushmaster Match Target which was boringly sweet at 1/2 MOA. A seriously all-day-long Good Thing.

I've also had a few rifles which were excellent from the git-go. Not free-floated, not shimmed. The best one was a Ruger heavy-barrel .220 Swift. I never checked about the bedding, since from Day One it shot 3/8 MOA. I dunno; back around 1973 or so. For now, I'm happy with my Ruger 77 Mk II in .223. Showroom stock except for the Timney trigger. Five-shot half-MOA, most of the time.

I just figure that whatever works is good. :)

Jimro
March 6, 2014, 09:29 PM
Bart B.

Would you rather I use xylophone bars, or tuning forks?

A barrel is much like a tuning fork, except you shave off the bottom fork and replace it with a stock forend.

If you want to increase the pitch of a vibrating rod, put a dampening point at the halfway position, what this will do is give you a perfect octave. 1/2 the length, 2x the fundamental pitch. 1/3 the length (two evenly spaced pressure points) 3x the pitch, so forth and so on.

As you know, stiffer barrels resonate at higher frequencies. By putting pressure at some point, you are intentionally creating a harmonic at some multiple of the fundamental.

Barrel blocks work in a similar manner, they totally encase the barrel to some distance forward the action, and so you can free float the rest of the barrel which will resonate at a higher fundamental frequency (be stiffer) and also free float the action. The difference with a pressure bed is that it maintains contact with the stock at the action and at the forend. Of course barrel blocks also take the majority of tension off the action threads, which is very important with very heavy barrels.

As a service rifle shooter, you have probably seen rack grade rifles that shot very tight with the sling used, but you put those rifles in a rest and the groups open up. The sling is acting as a damper on the barrel, which is "tuning" the barrel forward of the sling position to a higher frequency.

Conservation of energy tells us that the amplitude of the higher frequency will decrease (more deflections back and forth in the same time means less distance between peaks and troughs compared to fewer oscillations in the same time which means more distance between peaks and troughs).

But if I wasn't clear before, pressure bedding is not an optimal way to get accuracy from a rifle. If free floating doesn't work, pressure bedding is the what you try before going to a rebarrel. Once you put the stock back into contact with the barrel, it is tough to maintain consistent pressure at that point which produces consistent harmonics, which is why it is a technique pretty much reserved for hunting rifles that aren't expected to shoot long strings.

Jimro

jrothWA
March 6, 2014, 11:41 PM
my M70 deer rifle was bedded as a free-floater, @200 prone and 165gr Sierras, SPBT ot HPBT I'll group three shots within the "X" of standard 200 target.

Made a new stock for Anshltuz1407/11 , pillared bed the screws and bedded the two inches of barrel before the action, but as the bedding was thickening, I taped the barrel and stock and INVERTED the entire unit.

This allowed the barrel to drop away and the bedding compound to form around the downward pulled (gravity) barrel, when hardened (24hr later), the barrel has a constant upward force and results in set vibration pattern.
Used it new stock to group ammo and found the group tighten.

Have couple of Win M88 & 100 rifle that I bedded the action and pressured pointed the barrel ( after free floating) at the front barrel screw tendon and again getting great group on the "X" @ 200, using same reloads as the M70 used.

Bart B.
March 7, 2014, 10:39 AM
J. Roth, does your stuff and you shoot good enough to see the small change in shot impact vertically from that bedding pad under the chamber area of your barrels? The barrel does expand with heat as does the bedding under it and that changes the force of the bedding against the barrel. It's more noticable with centerfire rifles than rimfire, but it happens with both.

If the barrel's not fit right to the receiver, that might mask the vertical shot stringing from barrel pressure on that bedding pad because the barrel's bending from that misfit to the receiver.

reynolds357
March 9, 2014, 07:53 PM
Free floating is definitely the way to go on a quality build. Pressure points will make a poorly bedded rifle shoot better in many instances. With a tupperware stock and a poor bed job, pressure point will more than likely outshoot floated. With an aluminum block in a quality stock, floated is definitely the way to go.

Bart B.
March 10, 2014, 08:24 AM
Jimro, I'd rather you used something shaped and held like a rifle barrel. Round and thinner at one end tapering to thicker at the other end that's fixed in place. The back half of a barrel is thicker and heavier than its front half. And the end of the back half is fixed while the end of its front is free. No way will each half of a barrel whip the same if a pressure point's put mid point. That point will also move around unless something external to the rifle holds it fixed.

Note how the stock's fore end whips vertically when s rifle's fired in the link below:

http://www.varmintal.com/amode.htm

There's no way a pressure point on a barrel at the stock's tip stays the same when a bolt action rifle's fired hand held in any position. Also proof that any external force at that point will change the pressure and its force axis on that barrel. If there was a pressure point on the barrel from the fore end tip when it fired, when both the barrel and stock part company as they flex, the barrel's vibration and whip characteristics will change.

M1 and M14 rifles tested in accuracy cradles for accuracy by the service rifle team shops showed different amounts of accuracy depending on how much down pull the barrel had to the stock by the fit of the barrel band to the stock ferrule. They shot most accurate from that free-recoiling machine rest with about 30 pounds of down force for M1's and a bit less for M14's. The best barrels in them shot good lots of commercial match ammo into well under 2 inches at 300 yards; clip (magazine) after clip (magazine) and so on. And grouped each string of fire on top of each other. Nobody shot them that well off their shoulders and had to change zeros with different amounts of sling tension when slung up in prone. That sling tension changed the force on the barrel through the stock's fore end where the ferrule held the barrel band down. I've been there and done that with super-accurate 7.62 Garands.

Bart B.
March 10, 2014, 09:22 AM
My Security Group's declassified my secret.

Put something hard in the stock's fore end tip and the barrel to put pressure on the barrel.

Put an optical collimater in the barrel's muzzle, lock it in place, mount a high power scope on the rifle, zero it on the collimiator reticule. Then look through the scope in all sorts of shooting positions and watch the scope's reticule move about the collimator's reference. Click the adjustmets to put the reticle back to its original place on the collimator, counting them to see the amount the barrel bent.

Point the rifle straight down, note the scope's reticule on the collimator, then move the rifle back up to horizontal. You'll see how much the barrel bends its muzzle axis due to its weight alone. Yes, the collimator adds a bit of weight, but not a significant amount.

Best thing to do is bench the rifle resting its fore end on something then change your shooting positions and pressure the rifle's held on its rest and see how much the collimator reference moves around the scope's reticule. If the barrel's totally free floating, little, if any error will be seen.

You can use a laser bore sighter, but an optical one's easier to see the smaller changes.

Jimro
March 10, 2014, 09:45 AM
Bart B.

I'd rather you used something shaped and held like a rifle barrel. Round and thinner at one end tapering to thicker at the other end that's fixed in place. The back half of a barrel is thicker and heavier than its front half. And the end of the back half is fixed while the end of its front is free. No way will each half of a barrel whip the same if a pressure point's put mid point. That point will also move around unless something external to the rifle holds it fixed.

No duh. You argument that each half won't "whip the same" is pointless, and let me explain why.

When you put pressure on a vibrating object, you are dampening that object at a specific point. In a musical instrument, like a guitar string, the primary tone is produced where the most energy is released, and it is designed to be over the body of the guitar and not the neck.

Putting pressure on the barrel from the stock does the same thing, dampens vibration and change the pitch of vibration.

And not to be a jerk and make you read your own link, but I'm not the only one using musical instruments as an analogy.

Put another way, consider a guitar string being plucked. One pulls the string into a position (forced position) then releases it and the string vibrates at is natural frequency. The recoil and bullet motions "pulls" the rifle barrel to a new shape and once the bullet leaves the barrel, then the barrel vibrates. However, the addition of the scope to the model has shown some small high frequency vibrations superimposed on the forced deformations, both of which, slightly alter where the muzzle points before the bullet exits. For lowering the amplitude of the high frequency vibrations, it appears that even an "out of tune" tuner is better than no tuner at all.

Jimro

Bart B.
March 10, 2014, 11:01 AM
Jimro, I understand what you're saying. But my words you're quoting refer to a similar vibrating thing, not the same thing. Barrels and instrument strings are not held the same way; one fixed at one end. And a barrel with a pressure pad can move free of that pad when it wiggles before the bullet clears its muzzle. And the pressure on that point is not repeatable in amount and direction in all shooting environments. Neither has the same shape as they vibrate. Nor will they have the same shape if a pressure point is placed somewhere in their middle area.

My example was used to show how other long round things are shocked into vibration. Never intended to infer they both behaved physically the same way because they are not mounted the same way.

Jimro
March 10, 2014, 01:20 PM
Bart B.

The string is a useful analogy. All analogies convey an idea about what it happening, not the ultimate truth about what it happening. That is why people keep bringing up musical instruments.

In a rifle barrel the barrel will vibrate with a fixed point at the receiver and another fixed point at some position in the barrel towards the muzzle, as measured by Bill Calfee here: http://ozfclass.com/articles/1/psm_2005_03.html

But I will try to explain pressure bedding using only a rifle barrel as an example.

By putting a dampener on the barrel with stock pressure, the barrel is going to vibrate less simply due to energy absorption by the stock, but also shift the remaining "free" portion of the barrel into a higher frequency which has a dead spot closwer to the muzzle. You would need do do some Fourier Transform math to calculate the frequency based on measurements, but it could be done (to separate the fundamental frequency that was dampened from the resultant higher pitch frequency).

As the metal heats up, the effects of pressure will change as the barrel steel changes plasticity so accuracy over the long range will suffer. Free floating barrels are isolated from outside influences, so as long as everything is mechanically true, there won't be a loss of accuracy even with a hot barrel. Group size may open up slightly (mainly due to friction), but the group statistical center should remain the same.

Jimro

Bart B.
March 10, 2014, 07:56 PM
Having shot many rounds in several 316R stainless steel and 4440 chrome moly barrels at long range going from ambient temperatures (30 to 100 degrees F) to 150 -170 degrees F without any noticable change of bullet impact, I don't think decent barrels change their structural properties enough from temperature lowering their stiffness enough to make it visible on target. I've looked at tables showing the change going from 50 to 150 degrees and it takes up so little space on the graph's line it's indistinguisheable.

I'm going to contact the guy at Vibration Data who wrote the software to calculate barrel stiffness about that. And if it's important, ask for a software upgrade to plug it in and see the effect. Also ask him if he would put in a fluting option defining the width, depth starting point near the muzzle, length and number of flutes. That way one could easily tell how much fluting reduced a solid barrel's rigidity. With the influx of more fluted barrels on the market, he may well sell more copies of his software.

243winxb
March 10, 2014, 09:47 PM
A thin barrel, most times, will be more accurate with 9 lbs up pressure from the forearm. I glass bed them in after testing both free float & pressure point. Your result may be different. :)

Bart B.
March 11, 2014, 07:13 AM
What are the dimensions of a thin barrel for all calibers from 22 through 35 that makes a 9-pound upward force work so well?

How far back from the muzzle does that force point need to be for each caliber?

Bart B.
March 11, 2014, 09:05 AM
I had to read Bill Calfee's article a few times to make sure I wasn't duped. 'Twas hard to get by his starting every point with "Man,..." which reminded me of youngsters back in the '60's and '70's whose behaviour was subdued by depressant consumption; solid, liquid or gas forms.

Anyway, I find it hard to believe that a barrel's whipping and vibrating comes to a stop for 1/1000th second as he claims stating: The really accurate grouping from a target barrel comes when the hand load is such that it causes the partial cycle to be either at its highest or lowest point, because the muzzle of the barrel comes to rest for a millisecond at these two extremes.As that barrel's wiggling at several dozen (several hundred with harmonic overtones) cycles per second, where does the external force come from that makes it stop wiggling? And where does it come from that makes it start wiggling again?

And barrel time for most centerfire cartridges is between 1 and 2 milliseconds, a huge spread in muzzle veloctiy would easily put each one in that 1 millisecond dead/immobile time frame.

It sure contradicts Varmint Al's opinion based on physics:

http://www.varmintal.com/amode.htm

Art Eatman
March 11, 2014, 09:15 AM
Bart, if indeed a barrel vibrates like a tuning fork, then it stops in the same manner: A decay in the remaining energy that causes the vibration. Time? I don't know, but a barrel is much stiffer than a tuning rod.

Never had a barrel hum to me. :D

reynolds357
March 11, 2014, 12:24 PM
I take the "Japanese approach" to rifle building. I let someone else figure out what works and I copy it.

Bart B.
March 11, 2014, 12:36 PM
Do your use folks' ways that prove excellent over hundreds (thousands) of the best examples of accuracy?

Or those whose ways occasionally produce a few average examples from what used to produce a few horrible ones?

Jimro
March 11, 2014, 01:19 PM
Bart B.

As that barrel's wiggling at several dozen (several hundred with harmonic overtones) cycles per second, where does the external force come from that makes it stop wiggling? And where does it come from that makes it start wiggling again?

Not an exterior force, simple inertia. As the barrel moves towards position of maximum position velocity decreases towards zero until maximum position is achieved, then it goes back the other way accelerating until it hits the halfway point towards the other maximum position.

And barrel time for most centerfire cartridges is between 1 and 2 milliseconds, a huge spread in muzzle veloctiy would easily put each one in that 1 millisecond dead/immobile time frame.

Which gets into Dan Newberry's "OCW" load method, or the Optimal Barrel Time load method.

It sure contradicts Varmint Al's opinion based on physics:

No it doesn't. The theoretical harmonic nodes Varmint Al calculated were an attempt to understand how a barrel tuner works. Node 1 would be what happens when you "pluck" the barrel from the muzzle end, Node 2 is what Bill Calfee recorded in his test. Node 2 is what happens when you fire a cartridge, which is explained in detail here: http://www.varmintal.com/aeste.htm

Every material will have different resonant nodes depending on the input energy. You can see this with the classic "steel plate" experiments: http://www.phy.davidson.edu/StuHome/derekk/Resonance/pages/plates.htm

Jimro

Bart B.
March 11, 2014, 04:11 PM
Jimro, are the "Node" examples you refer to the "Mode" ones used in Varmint Al's pages? A "node" in vibrating rods, beams, barrels and strings is the point where no movement happens; the zero point the curved parts are on either side of as shown in the link below:

http://en.wikipedia.org/wiki/Node_(physics)

If so, note all those modes happen at the same time when the rifle's fired. Each higher mode number occurs at lower amplitudes but higher frequencies. Mode shapes before and when the bullet exits are not the same as the eight modes are for the barrels natural vibrations after the bullet leaves.

Esten's barrel pointing curve with the tuner goes through one half cycle in about .0006 second. That's about 833 cycles a second. A high harmonic of that barrels resonant frequency. Some estimates of the initial frequency barrels whip at as the bullet leaves is 5 to 6 times it's resonant one.

You're making me think!!!! Thanks!!

Jimro
March 11, 2014, 06:56 PM
Bart B.

If so, note all those modes happen at the same time when the rifle's fired. Each higher mode number occurs at lower amplitudes but higher frequencies. Mode shapes before and when the bullet exits are not the same as the eight modes are for the barrels natural vibrations after the bullet leaves.

First off there are more than 8 modes, there are actually infinite modes. This is called an "overtone series" to musicians http://en.wikipedia.org/wiki/Overtone but the exact same thing happens with any sort of vibration. Secondly not all of those wave modes have to happen when a cartridge is touched off, as some of them are caused by the primary harmonic having interference. So yes they will show up, but not always right away.

But as you noted, the higher the frequency the less physical effect there is to measure. This is a logarithmic relationship http://plus.maths.org/content/perfect-harmony which means if you are looking for answers, it doesn't lie in the third or higher harmonic.

As Varmint Al noted, the fundamental being less than 400 Hz won't have time to complete a cycle before the bullet leaves the barrel. That leaves the first harmonic (2nd Mode) as the "tuneable" mode for the barrel tuner, where they tried to move the position of the tuner to a point where optimal muzzle angle on bullet exit was achieved. What they really pointed out that was having added mass on the muzzle was a good thing, even if the tuner was out of tune. Remember if the primary harmonic has a value of 1, the secondary will be at least half that, and the third harmonic at least half the second.

Now the optimal barrel time theory of reloading http://www.the-long-family.com/OBT_paper.htm does rely heavily on "Mode 4" to explain accuracy (the "moving bulge" explanation), something you have commented on before about Garands still shooting tight even after there was no copper wash on the rifling near the muzzle. Assuming an even bore internal diameter, this makes sense, although it also makes sense when you note that barrels with decreasing bore diameter towards the muzzle have also been known to be very accurate.

Still, Varmint Al and Bill Calfee agree that the "Wave Mode 2" is how rifle barrels bend by calculation and experimentation.

Jimro

splatman
March 17, 2014, 08:32 PM
Also keep in mind that pressure on
the bottom of the receiver will vary
with a floated barrel depending on
the location of the rest in relation
to the forearm. Also big heavy bull
barrels can stress the receiver if
the barrel is completely floated.

splatman

Bart B.
March 17, 2014, 11:45 PM
Splatman, long heavy barrels have hung free floated from one of the least stiff, easily bent commercial actions since the 1960's. Shooting as accurate as modern single shot very rigid actions do these days with the same size barrels.

All receivers bend some amount regardless of what the barrel weighs. But it's the same for every shot fired.

splatman
March 18, 2014, 08:04 AM
Actually receivers bend more under heavy
bull barrels when completely floated.
Custom actions will usually be
threaded deeper to support such
barrels. Glass bedding also helps
to reduce the amplitude of the
harmonic way. Keep in mind that
that although pressure points will
mitigate the amplitude the frequency
of the wave will increase. This is
usually preferred as it makes fine
tuning a load more easily done. I
do recommend completely free
floating a wood stock since wood
swells with humidity. For fiber glass
stocks I usually free float the barrel
and then glass bed the action and
the barrel since fiber glass is
completely stable. I would treat a
synthetic stock similar to wood.

splatman

Bart B.
March 18, 2014, 09:17 AM
If you think fiberglass stocks are perfectly stable, you must not be familiar with the problems fiberglass stocks had on match grade M14's and how they bent from sunlight on one side. Zero's also changed from hot to cold ambient temperatures. They were no different that synthetic ones. Their fore ends bent enough to change the pressure on the barrel at its band the stock ferrule pressed against. But the wood stocked ones had no problem.

Both non-wood stock material types change dimensionally and structurally with temperature.

All stock fore end's bend from external pressure some amount relative to the axis of a receiver solidly fixed to them. It's the most at the fore end's tip and much less close to the receiver and it depends on the size and material, too. The axis and amount of that pressure depends on how the stock's held and steadied by the shooter. If you measure how much they do bend, you may well be surprised. There's no way any pressure point from the fore end to the barrel will remain constant in amount and axis across all shooting positions. In some postions, it varies more than others. And that variable gets transferred to the barrel so it's not going to behave the same across all positions.

So do you think a 30 caliber, 26" thick, heavy target weight 4.4 pound barrel's stiffer than a 30 caliber, 22" thin, featherweight 2.1 pound sporter barrel and the feather weight one should have a pressure pad on it from the fore end?

splatman
March 18, 2014, 08:12 PM
Bart,

Yes I agree that putting pressure
on the forearm at different locations
will affect the pressure on the barrel
at the point where it is glass bedded
thus an inconsistency is introduced.
Likewise putting pressure on the
forearm at different locations will
affect the pressure at the point
where where the stock is secured
to the action. This pressure is actually
torque caused by a force applied to
a moment arm. There is actually
more torque on an action when a
barrel is completely floated. It is well
known that glass bedding the barrel
a few inches in front of the lug greatly
improves accuracy. If you have a
heavy 1.25 inch barrel with a straight
contour and it is 33 inches long you
have some major weight to support, in
such a case I would also recommend
glass bedding a few inches at the far
end of the forearm. If done correctly
these contact spots will transfer the
energy from the shock wave into the
stock. So the bottom line is that free
floating a barrel does not alleviate
the problem which occurs when
pressure is applied to the forearm
at different locations.

splatman

Bart B.
March 18, 2014, 08:34 PM
The most accurate, match winning, record setting, receiver bedded rifles in all disciplines have nothing touching their barrels except the receiver. Their owners have oft times proved to others that bedding under the barrel causes vertical shot stringing. But if you and your stuff isn't up to the precision to show it, then you'll never understand.

If you shoot your barrel-bedded stuff into no worse than 1 inch at 300, 3 inches at 600 or 6 inches at 1000 for 20 to 30 shot strings in as many minutes, then I'll be interested.

splatman
March 18, 2014, 09:16 PM
Sorry not all the top shooters float
their barrels. I don't think you
understand... its not whether you
float your barrel or not. Its all
about consistency in managing
your rifle and your loads.
splatman

old roper
March 19, 2014, 06:52 PM
Sierra uses one of these to test their bullets

http://www.benchrest.com/magnummetal/

Barrel sure not free floated

In order to accomplish accuracy firing tests, Sierra utilizes our 300- meter underground testing range daily. 200-yard accuracy evaluations are conducted on rifle bullets using a precision unrestricted machine rest. We also conduct daily 50-yard accuracy evaluations on our pistol bullets using a specially designed machine rest.

Bart B.
March 19, 2014, 07:58 PM
The action is free floated. You gotta float something. It's used for just testing bullets for quality for Sierra Bullets.

Sierra's earlier rail guns were a simpler design with fewer parts. They used Savage actions earlier and Remington ones before them.

Some folks in the late '60's epoxied a 6" long near 2" square split aluminum block in wood stock fore ends clamped on match rifle barrels with the action and box magazine floating in the stock. Accuracy with .308 Win barrels was no better than conventional epoxy bedding receivers with the barrels free floating.

Jimro
March 21, 2014, 01:10 PM
True, but a barrel block also produces no worse accuracy than traditional epoxy bedding, and is easier to do if you have a machine shop to build the block.

The biggest advantage of a barrel block is that you can get away with using heavier barrels than the receiver was ever designed to support, because the receiver only has to support its own mass as the barrel is supported by the block. You see this fairly frequently with Ruger 10/22s that have been tricked out with a large bull barrel and unlimited class benchresters.

Jimro

Bart B.
March 22, 2014, 08:52 AM
Take a look at this Mann rest used to test .30-06 ammo near a century ago. Good lots of match ammo back then shot as well from it as it did in the 1960's when it was discontinued at Lake City. It's shown in post 15.

http://forums.thecmp.org/showthread.php?p=88475

mete
March 22, 2014, 02:41 PM
In gunsmithing school in the 70s we were taught to completely bed the receiver + 2" of the barrel .This for typical hunting rifles at least.
My last gun purchase was a Kimber syn/SS supposed to be free floating but wasn't .I made it free floating which dropped the groups in half ! I'll take it !

Vibration and their problems can be frustrating .A few [non-gun] problems I've dealt with defied all logic and reason! :eek:

Bart , it's spelled 'metallurgy ' :)

Bart B.
March 22, 2014, 08:48 PM
mete, I know how it's spelled. Sometimes my fingers don't double tap a key when they should.

And it's my opinion that several myths about firearms have been passed on to 'smith students be well meaning teachers who are ignorant of realities and ignore the facts that counter their beliefs.

Jimro
March 24, 2014, 02:31 PM
In gunsmithing school in the 70s we were taught to completely bed the receiver + 2" of the barrel .This for typical hunting rifles at least.
My last gun purchase was a Kimber syn/SS supposed to be free floating but wasn't .I made it free floating which dropped the groups in half ! I'll take it !

Mete, you learned on an M98 action?

Jimro