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Old March 7, 2013, 09:19 PM   #17
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 4,408
Unclenick, meplat uniformers only clean up the outside of the hollow tip of bullets. They do nothing to the inside. As hollow points formed by a pointing die swaging down and shaping the ogive from a straight wall lead cored jacket, they are not uniform in their dimensions all the way around. Sierra Bullets' HPMK meplats look like the mouth of a volcano and there are no perfectly-shaped volcanos on this planet. They show the imperfect properties of the jacket material as it's swaged down to the shape of the front half of a football. Cleaning up the outside does nothing to the inside. And everything behind where the uniformer removes metal is still like it was before. The lead core and the rest of the bullet jacket still have their dimensional variables. So, the rest of the bullet is still unbalanced as it's center of mass ain't exactly on the center of form. Cleaning up the meplat may reduce bullet unbalance as well as change the airflow around it a small percent, but I think it's hard to verify. But some people swear (with nice words, not dirty ones) by them.

One could measure the bullet unbalance like Mid Tompkins did back in 1971. In company with a machinist at Sierra Bullets, they made a well balanced collet to fit in a Dremel Moto Tool with a cup shaped to hold Lapua D46 185-gr. FMJRB match bullets. They'd already sorted several boxes of them by shape on an optical comparator; found four distinct shapes, obviously from four different forming dies. Spinning those bullets at 30,000 rpm and measuring current to the Dremel's motor showed more current needed for more unbalanced bullets, less for the well balanced ones. More current was needed to keep the motor spinning when the centrifugal forces of unbalanced bullets put more drag on the motor's bearings. In actual firing, those bullets would spin near 6 times as fast. A few were so unbalanced they flew out of the collet and bounced off the ceiling and walls. But they shot several groups at 600 yards all under 1.5 inches; some well under an inch down to .7 inch. .7 inch is a bit over 1/10th MOA at 600 yards. Today's benchresters don't shoot their stuff that well. And Mid used his Hart-barreled pre-'64 Win. 70 action in a solid wood stock shooting WCC58 .308 Win. full length sized cases with about 42 grains of IMR4064 powder in a SAAMI spec chamber.

Sierra Bullets' reloading manuals have published time-of-flight data showing how unbalanced bullets effect drag. Some bullets entering the timing screens had different times through them but entered at the same velocity. Therefore the slower ones had more drag. Wasn't much, but a few microseconds makes a difference. They calculated BC varied 1 to 2 percent because of the amount of unbalance the bullets had.
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Last edited by Unclenick; October 16, 2013 at 08:36 AM. Reason: fixed a typo to make the post easier to search for
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