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Old November 5, 2012, 07:52 PM   #23
Jimro
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Join Date: October 18, 2006
Posts: 5,450
I agree with Bart B. about not buying into the "shockwave in steel" theory.

I do however believe in barrel harmonics (in the sonic range, just like a church bell) and timing to coincide with that frequency.

How I understand harmonics: round steel rifle barrels vibrates in a "figure 8 pattern" with one major orbit and one minor orbit (in a perfect system the orbits would be equal, but normally we fire rifles relatively horizontal to the gravity well). The "shockwave" in the steel itself is traveling 90 degrees through the steel from this figure 8 vibration has very little to do with this other than providing the intial "hammer strike" that "rings the bell"

The muzzle whipping around in that figure 8 pattern will spend some amount of time in each orbit (if I remember physics correctly the velocity changes with the radius of orbit), so obviously you want the bullet to exit in the minor orbit (less movement because of a smaller distance to travel, so the actual velocity of the muzzle it as its smallest) even if the barrel spends a smaller amount of time in that "node."

As I understand it, OCW is about timing your load to always have the bullet leave the barrel on the minor node of the figure 8 (which is the physical vibration of the barrel itself, nothing much to do with the shockwave that started it).

This is why loads that have the same barrel time and different velocities can both group well, or you find distinct "accuracy nodes" during a load workup (For a 308 normally around 2,400 fps, again at 2550 fps, and again at 2650 fps, and 2750 fps (with a 168 or 175 bullet), and again at 2900 to 3000 fps for long barrels with 155 gr bullets).

This can also explain why the load with the lowest SD and ES may not be the tightest grouping in your rifle all the time (though generally it is).

But I don't have the experimental equipment to actually run the tests I'd need to confirm or deny my understanding of the whole process.

Jimro
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