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Old November 18, 2012, 07:42 AM   #10
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 6,318
Get a couple of drill bits to use as hole or "pin" gauges. A #45 is .0820" and a 5/64" is .078125".

If the #45 won't go into the flash hole but a 5/64" will, they're good for tolerance. If the 5/64" won't go in the hole's too small for specs. Conversley, if the #45 does go in, the hole's too big for specs.

I've made 20-shot test groups comparing factory flash holes that varied about .003" in that range to cases that had them all drilled out to .086" with a #44 drill bit. No difference in accuracy whatsoever.

Therefore, I don't think one needs "exact" flash hole diameters. It's my opinion that anything over .086" will start causing high pressure problems; the powder's going to ignite faster and the peak pressure may well be higher.
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