Home Forums Hunting & Shooting Ask The Gunwriters Primer Flash Hole Size???

I was cleaning several thousand 45 ACP cases today. The cases were a mixed lot of Federal, Winchester and, Remington.

As I pushed the cleaning media from the primer flash holes, I noticed that some flash holes were noticeably larger than others. I sorted 10 of them into large and small hole groups and using drill bits and a micrometer, I measured the hole diameter.

The Winchester and Remington case's holes measured 0.075".
The Federal case's holes measured 0.095", or 0.020" larger.

I assumed that SAAMI-adopted standardized specifications would cover flash holes. Can anyone shed some light on this?

With a larger flash hole, would the effect of the larger hole on the flash reaching the powder be different? What might be the effect on pressure?

Any ideas --- anyone?

Yes, different-size flash-holes will result in different pressures. Usually larger flash-holes create more pressure, because more of the powder ignites sooner.

I've always thought flash-hole size would be SAAMI-standardized as well. Did you fire all this brass new?

This sounds dumb to write, but are you sure all of the brass had large primer pockets? I'm aware of some 45 ACP brass that is now using small pistol primers.

Do a little searching folks and there's any number of somewhat mutually contradictory stories out there.

The drive for less lead on pistol ranges includes changes in the primers. The idea is to balance the weaker flash from a less toxic primer with a larger flash hole for substantially similar ignition. See also the move to small primers. See also Bill Jordan's discussion of enlarging flash holes for wax bullets then segregating the cases compared with the current general experience that larger and common size flash holes don't matter much for normal use on a square range. CF with the NT marked cases and small primers.


From a Tom C

Now gone from the website with the move to small pistol primers Winchester once said:


Others say the Federal at least will have loose primer pockets sooner than the conventional size flash holes. Some worry about powder sifting back into the primer or otherwise having more leaking primers and breechface erosion. Some say the Speer Gold Dot as optimized for short barrels uses a slightly enlarged flash hole.

On the other hand the PPC folks demonstrated pretty conclusively that ignition matters and varies with primer flash hole size as with primer size. See also Jim Carmichael's efforts with small rifle primers in the .308 case head and so it goes.

No John, some was range pick-up; most was fired by me. I too assume that larger flash holes will result in larger pressure as there will be a larger spark contacting the primer in a shorter time period.

That was my first thought; all are large (pistol) primer size. I believe that Federal uses a small pistol primer in their "white box" ammo.

I once did a test with RWS 5,6x50R brass. The flash holes were very precisely drilled but looked very small diameter and I opened them up to the regular size by using a Lyman tool. The results? Velocity increased by 30fps or so, extreme spread doubled, and groups opened up from .5-.7" to 1".
BBerg