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Seperating by bearing surface?

 
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Old 09-09-2012, 07:18 PM
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Re: Seperating by bearing surface?

I've had no problem getting Sierra's 30 caliber HPMK's of 155, 180, 190 and 200 grain weights right out of the box (no measuring in any way) to shoot under 5/8 MOA all the time (in good conditions when tested with 15 to 20 shot groups) at 1000 yards.

One person took a bunch of Lapua 185-gr. FMJRB 30 caliber match bullets and spun them in a tool that measured how well they were balanced. Some quite unbalanced flew out of the collet holding them in a Dremel Moto Tool spinning at 30,000 rpm. About half were perfectly balanced as indicated by minimum current needed to spin the motor; the more they were unbalanced, the more current was needed to spin the motor up to speed. Those perfect ones were shot into several 10-shot groups at 600 yards; they ranged from about 1.5 inch down to under .7 inch. A 40-shot test of them went into 1.92 inch; about normal compared to what the 10-shot groups' sizes were. Before spinning them for a balance test, they were put in an optical collimator that showed slight shape errors in the ogive and rebated base as well as length overall. That didn't seem to matter as it' normal when the same dies coin, cup, draw and core with lead as pure as can be, shape a sheet of jacket material that's never perfect in its metalurgy makup and dimensions.

Most match bullets have a slight unbalance that causes their BC to vary 1% or more as they wobble different amounts in flight. Only bullet spinning true on their shape axis have the lowest and most constant BC. The more they wobble, the more BC drops. And a 1 to 2 tenths grain in weight change is negligible as far as BC is concerned.
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