Free float or pressure points

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
 
Both is the magic answer..

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.
 
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.
 
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.
 
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.
 
Last edited:
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.
 
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.
Click to expand...

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.
Click to expand...

Jimro
 
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.
 
Last edited:
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
 
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.
 
Last edited:
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. :)
 
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?
 
Last edited:
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.
Click to expand...
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
 
Last edited:
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
 
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?
 
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?
Click to expand...

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.
Click to expand...

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:
Click to expand...

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
 
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!!
 
Last edited:
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.
Click to expand...

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
 
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
 
You must log in or register to reply here.
Back
Top