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Old November 5, 2012, 04:20 PM   #22
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 5,389
OK, I get that, it's the "heart" of the OCW principle.
Well, sort of, regarding when bullet leaves the barrel at the right time.

The OCW theory's based on the assumption that best accuracy's when the shock wave from the round firing has gone forward to the muzzle and is back at the breech end when the bullet leaves the barrel. As nobody's ever hooked up test equipment to verify this is reality, it's still just a theory. Meanwhile, consider the following facts; yes, facts.

The speed sound waves propagate in steel is stated somewhere between 13,000 and 19,000 fps depending on the source one looks it up in. Whichever speed you choose to use, it means that shock/sound wave from the cartridge will take from .000222 to .000308 seconds for one round trip back and forth in a 24 inch barrel. Typical barrel times for a .308 Win. bullet to exit a 24 inch barrel's about .002500 seconds. That shock/sound wave will make about 8.1 to 11.2 round trips from breech to muzzle while the bullet's going down the barrel. So when the bullet exits, one of those sound/shock waves will be no more than a few inches back from the muzzle. Until it's defined as to where that shock/sound wave starts at, there's a 2 to 3 inch range of barrel length at the back end one has to start its timing from; who knows where that's at? And nobody's published any inof on how long the bulged area of the barrel is where that shock wave is at. Nor how much does it enlarge the muzzle.

OCW believers championing the theory that the shock/sound wave makes the muzzle larger and that's a bad time for the bullet to exit might consider the following. Folks have tested top quality match conditioned M1 service rifles whose bore's been rubbed bigger by steel cleaning rods slid back and forth in their muzzle. The lands and grooves are enough larger that no copper wash from jacketed bullets exists for the last 3/4" of the bore. Yet they still shoot under 2/3 MOA with good lots of both Federal GMM .308 ammo as well as good lots of National Match lots of M118 or M852 arsenal ammo. That's as good as they shot 2000 rounds earlier when the barrel was new. Many very accurate barrels will have a groove diameter .0001" larger at the muzzle than a point a few inches back from it; that's normal best-qualty barrel making.

Yet in barrels from 22 to 28 inches long with a variety of bore and groove diameters shoot Federal GMM .308 ammo very accurate in spite of having that shock/sound wave all over the place in them as well as lots to cycle times for that wave to round-trip in those barrels along with lots of barrel times for each.

OCW theory, in my opinion is far too much UWAG-ing for me to believe. I'm still waiting for someone to prove it's reality. But it's a good competitor for Browning's BOSS system the makers thereof claim it makes the barrel shoot best (again, no testing's been done to prove it) when the bore straight out in the middle of its whip cycle; no mention of when its on the down or up swing when that happens.

getting the bullet to exit the muzzle at it's most consistent point in space?
I don't think either of those two places where the barrel's most consistant point are best for accuracy. Here's why. But first note that most barrel's whip frequency at their muzzle is 3 or 4 times their resonant frequency or a few hundred cycles per second. They'll go through one cycle in about .002 to .004 seconds or something like that.

There's two of them; one at the top of the muzzle axis whip arc and the other's at the bottom. Barrels are bent the most in the wiggling whipping cycle at that point. But how much that angle changes in a few millionths of a second is the least at those places. Note that the barrel time from case mouth to muzzle will have a small spread which causes a small spread in muzzle velocity. Getting bullets to leave at either the top or bottom one brings up the following situations...

If the bullet's leave about the high point, faster ones will leave sooner as the angle gets bigger just before the muzzle axis is at its highest angle. Slower ones will leave after the axis is at its highest angle while it's on the way down. Some slower ones will exit at the same angle as some faster ones.

If the bullet's leave about the low point, faster ones will leave sooner as the muzzle angle gets lower just before reaching its lowest angle. Slower ones will leave after the axis is at its lowest angle while it's on the way back up. Some slower ones will exit at the same angle as some faster ones.

Both of the above will cause too much elevation shot stringing; each bullet's muzzle velocity's not compensated for. However, if all the bullets left just before the muzzle axis reached its top, they all would have good compensation to correct for bullet drop. Faster ones leaving sooner at lower muzzle axis angles would have about the same impact point as slower ones leaving later at higher muzzle axis angles.

Last edited by Bart B.; November 5, 2012 at 05:17 PM.
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