Free float or pressure points

[chipchip]

Why do some rifle manufactures free float thier rifle barrels while others use a pressure point. Is one better than another.

[iraiam]

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]

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.]

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]

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]

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]

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.]

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]

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.]

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]

BartB- Stupid question here, but how do you measure that? With a straight edge or something?

[Bart B.]

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.]

baddarryl, Security Group's cleared you to get my top secret stuff. PM's been sent to you.

[Jimro]

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]

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.





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.




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.



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



This is a post bedding target.




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]

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]

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]

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.]

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]

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]

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]

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.]

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]

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.]

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.