Twist Rate

[Bart B.]

Unclenick, good example of the vertical drag's details.

When shooting 190's from a .308 through a chronograph on their way to 1000-yard targets, it was interesting to see some of the slower ones impact higher on target than faster ones. Good evidence that there's a very small spread in the BC of a given bullet. Even super-accurate match ones.

[Unclenick]

Bart,

Have you tried either meplat uniforming or one of the Whidden pointing dies to see if either one or a combination of the two addresses that well enough? I'm curious whether it's the tip or the irregularities you see in exact ogive radius that's primarily responsible. The latter could be uniformed by sorting.

[Bart B.]

Nick, no, never tried pointing or ogive sorting. Whidden wants sub 3/5ths MOA accuracy (as I remember from reading one of his articles some time ago) with his 1000 yard Palma ammo and he gets that using his pointing tool.

I often think it's more of a bullet balance issue; the more unbalanced they are, the more drag they'll have and they slow down faster striking lower. Having shot Sierra HPMK 155's, 190's and 200's that accurate at 800 and 1000 yards without point mods nor shape sorting from bullet lots that grouped well into the low ones as tested at 100 or 200 yards, and not through a chronograph to check velocity vs. elevation, I think those bullets are better balanced (uniform metalurgy and dimensions) that make that happen.

The only way to check balance is dynamically; you've got to spin them at least 30,000 rpm and measure their wobble. Those with the most wobble will jump the most off the muzzle axis in any direction when they exit. That reduces accuracy in all directions.

[Unclenick]

Bart,

I've not been able to talk myself into believing those processes will make much difference, either. Just your previous mention of variation in BC made me think to ask if it can be solved that way.

If the issue is balance, you're correct that spinning is the way to go, though there is (or was) a hand made $800 machine that checked bullets ultrasonically for internal imperfections. Harold Vaughn's book has an appendix that includes enough details to make an air bearing spinner that he used with dynamic headphone elements acting as sensors to sort bullets for wobble. That device only ran bullets up to 143 rps, but with with those sensitive elements it was enough. He also shows the static method using a torsion pendulum, but concedes it is slower and less accurate.

[Bart B.]

Nick,

Back in 1971, Mid Tompkins had a collet made by Sierra Bullet's tool and die machinist to fit the ogive profile of Lapua's D46 185-gr. FMJRB match bullets. The collet was chucked into a Dremel Mototool with an ampmeter in its power cord. Spinning those bullets at 30,000 rpm, here's what they learned.

The more unbalanced a bullet was, the more current was needed to spin them; the centrifugal forces they had loaded the bearings and more current was needed to spin the motor up to speed. Perfectly balanced ones drew the least current. A few bullets were so unbalanced they flew out of the collet and bounced off the walls and ceiling of the room they were tested in. Out of a thousand or more tested, he got a couple hundred "perfect" ones to shoot.

He shot a hundred or so from his machine-rested Hart-barreled Win. 70 using full-length-sized WCC58 .308 Win cases at 600 yards early one morning. His load was 42-gr. of IMR4064 with bullets seated to touch the lands. Several 10-shot groups all printed from about .7 to 1.5 inches (1/4 MOA at worst). One at about .8 inch (about 1/12 MOA) was pictured in a fall issue of the NRA's American Rifleman with a advertisement for Lapua bullets. A 40-shot group starting with a cold barrel printed 1.92" (under 1/3 MOA) so he decided to take the rest to the Nationals that year. Not too shabby at all considering the chamber was a standard SAAMI spec one.

[Unclenick]

Bart,

That's an interesting way to go about it. A trifle hard on the imbalanced bullets that go flying, but it sounds like it worked just fine with the good ones.

(Note: I typo'd in my last post; I had rpm instead of rps (8580 rpm), and have fixed that.)

Vaughn did an experiment¹ drilling a small hole into the side of a dozen flat base hollow point .270 bullets right at their center of gravity (so it would cause wobble in flight but not coning motion) that offset the center of gravity 0.00118". By keeping track of how the hole was oriented when seated, he was able to index the hole in finished rounds at 90° intervals during firing and watch where they hit. Firing at 100 yards, he showed each group of 3 shots with the same orientation was in the range of 1", with that 1" centered at about 12:00, 3:00, 6:00, or 9:00, depending how the rounds were placed in the chamber. These four 3-shot groups, were each offset about 2.5 inches from the average center for all 12 shots, giving about a 6" overall group size (2.5" mean radius for all 4 of the 3-shot groups + 0.5" for the individual group radii).

Vaughn also gives a formula for calculating the effect, which predicted 2.58" radius of dispersion at 100 yards. Pretty close to the 2.5 he actually got and within expected error for the sample size. That's about 1 inch of radius (2" group) for each 0.00046" of CG offset. He then gives a measured spread for the bullets (I don't spot mention of what brand they were) from the air spinner, and about 0.00014" was average, or a little more than 1/9 of his intentional drilled CG offset, and enough to cause groups to be 0.61" in diameter. These were obviously not match bullets. Out of 100 measured, he had four that were close to perfect and 7 that were around 3 times the average, with the worst one at about 0.0004" offset, enough so that two of them would make a 1.75" group if the light sides were oriented 180° apart. It's not exactly inspiring, but I've got a lot of pulled M2 ball bullets that do worse.

Vaughn's formula for the offset radius, σ, is:



Vaughn mentions that Dr. Franklin Mann also had a working formula for this that he'd tested and published in "The Bullet's Flight" in 1909, but says Mann's physical reasoning was incorrect by current understanding. Still, he calls it a remarkable book for its time.


¹ Rifle Accuracy Facts, Harold R. Vaughn, Precision Shooting, Inc. pub (RIP), 2nd Ed., 2000, pp. 169-180.

² Actual muzzle velocity used here is roughly 3% below instrumental velocity for a sporter length barrel due to the bullet gaining about 3% of velocity from muzzle blast after exiting the muzzle.