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Old April 15, 2012, 02:25 PM   #31
Join Date: March 4, 2005
Location: Ohio
Posts: 13,718
I can't imagine the burning surface area difference from a few damaged grains would make any measurable difference to performance any more than a little loose powder dust would. The main reason to avoid cutting grains is to avoid the jostling of the measure that occurs when the rotor snaps through a grain. I don't think I've ever had a cut charge weigh differently from the previous charge thrown, but I have had the charge right after a cut charge and sometimes the second one after come up a little heavy. The jostle rearranges the grains some.

As Bart says, the record setting powders are stick, though Glen Zediker likes 748 alright for service rifle matches. The fact the stick powders can work well despite a bit a charge variation has something to do with their ignition characteristics, and big sticks seem more immune than small ones. Hatcher's got an example. He wrote that he worked with two powders about like modern IMR4320 in burn rate when choosing the National Match load one year. One was short grain and the other was long grain. The Frankford Arsenal loading equipment could only meter the the coarse grains to a spread of 1.7 grains, but could meter the short grains to a spread of 0.6 grains. Nonetheless, in the test guns, ammo loaded with the long grain powder made consistently smaller groups at both 600 yards and 1000 yards, where you expect velocity variation to show.

I've thought before that perhaps as the sticks pack tighter the flame front passage between them becomes enough slower to compensate for a small charge increases. However, I don't see why that bulk density difference wouldn't just vibrate out during transport or change when cases were shaken or handled. So I expect there is actually something more complicated going on.
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