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Old March 10, 2014, 09:22 AM   #26
Bart B.
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Watching Barrel Bending from Stock Tip Pressure Point

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.
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Old March 10, 2014, 09:45 AM   #27
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Quote:
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.

Quote:
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.
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Old March 10, 2014, 11:01 AM   #28
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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.
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Old March 10, 2014, 01:20 PM   #29
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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.

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Old March 10, 2014, 07:56 PM   #30
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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.
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Old March 10, 2014, 09:47 PM   #31
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Wood Stock

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.
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Old March 11, 2014, 07:13 AM   #32
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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?
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Old March 11, 2014, 09:05 AM   #33
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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:
Quote:
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
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Old March 11, 2014, 09:15 AM   #34
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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.
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Old March 11, 2014, 12:24 PM   #35
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I take the "Japanese approach" to rifle building. I let someone else figure out what works and I copy it.
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Old March 11, 2014, 12:36 PM   #36
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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?
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Old March 11, 2014, 01:19 PM   #37
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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.

Quote:
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.

Quote:
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/...ges/plates.htm

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Old March 11, 2014, 04:11 PM   #38
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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!!
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Old March 11, 2014, 06:56 PM   #39
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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.

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Old March 17, 2014, 08:32 PM   #40
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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.

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Old March 17, 2014, 11:45 PM   #41
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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.
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Old March 18, 2014, 08:04 AM   #42
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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.

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Old March 18, 2014, 09:17 AM   #43
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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?
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Old March 18, 2014, 08:12 PM   #44
splatman
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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
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Old March 18, 2014, 08:34 PM   #45
Bart B.
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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.
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Old March 18, 2014, 09:16 PM   #46
splatman
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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.
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Old March 19, 2014, 06:52 PM   #47
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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.
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Old March 19, 2014, 07:58 PM   #48
Bart B.
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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.
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Old March 21, 2014, 01:10 PM   #49
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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.

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Old March 22, 2014, 08:52 AM   #50
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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
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