Thread: walking group
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Old September 7, 2012, 11:57 AM   #49
Bart B.
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Join Date: February 15, 2009
Posts: 5,385
MLeake, I know that. I explained why in post 5 for this thread:

Quote:
Barrels walking point of impact as they heat up is typically caused by the barrel is fit to the receiver with uneven pressure around where its flat part is against the receiver. If the receiver face is not square with the chamber axis, there'll be one point where the barrel is hardest against the receiver. As the barrel heats up and expands, a stress line at that point makes the barrel whip more and more in one direction as it gets hotter. Which is why, after letting the barrel cool down, it shoots back to point of aim where it started. This cause is easy to fix; face off the receiver and shim the barrel so it clocks into the same place keeping headspace correct.

If a barrel's not properly stress relieved, they also will bend a bit as they heat up. Replacing it is the only solution.
His barrel's fundamental/resonant frequency doesn't have anything to do with that.

Yes, I've heard of and seen the BOSS system on Browning rifles. Adjusting it changes the length of the barrel mass which changes the barrel's resonant frequency a small amount. It does not tune barrel vibration so a given load leaves the muzzle at a neutral point in the barrel's sine wave oscillation. There's no way anyone could verify that without accelerometers on the barrel feeding computer software along with bullet exit times to verify it did. If by "neutral point" you mean when the muzzle axis is straight out from the chamber, that's the worst place to have bullets exit. That's where the muzzle axis goes through the greatest amount of angular vertical swing in a given amount of time. There's no way one could get muzzle velocity spread low enough to do that consistantly. Also, if the muzzle axis is on the down swing as it goes through that point, faster bullets will leave at a higher angle and slower ones at a lower one; exactly the opposite of what's best. So one would have to ensure the muzzle axis is on the up swing when that happens. And muzzle velocity spread would need to be much smaller than what's acceptable for the best place in the vertical plane to leave at.

Browning claims their BOSS adjusts the barrel whip so bullets leave at the top or bottom point in the muzzle axis' vertical swing. I don't know if they proved that with equipment to acutally measure bore axis angle vs. bullet exit or just assumed this to be so like virtually everyone else making such claims.

It's best to have the bullet exit just before the muzzle axis reaches its highest point so some compensation for velocities faster and slower than the average would leave such that their trajectories would be corrected for. A very small spread in velocity is not needed. And this is what the BOSS system does if it's set correctly. This was proved over a hundred years ago with the British .303 ammo in their service rifles.

One can do the same thing much cheaper with the same results than a BOSS system does by just changing their powder charge weight a bit to change the bullet's barrel time from case mouth to out the muzzle.

With the right set of components in a given load, all the bullets fired from several barrels with different resonant frequencies will shoot sub 1/2 MOA at 600 yards with ease. That was proved years ago in long range competitive shooting. But for short range with .22 rimfire ammo, such "tuners" are popular and they do work well if one cannot find a lot of ammo that has the right barrel time for their barrel's vibrating characteristics.

Last edited by Bart B.; September 8, 2012 at 10:28 AM.
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