The Firing Line Forums - View Single Post - Flash Hole Size-Whats too large/small?

Bart B. · November 18, 2012, 08:42 AM

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.

November 18, 2012, 08:42 AM	#10
Bart B. Senior Member Join Date: February 15, 2009 Posts: 2,120	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.