primer flash hole size

[saed]

I bought new winchester brass (762.39) and noticed the flash holes are ruff, and even have small splits from the edges...
I was thinking of fixing them up, and then started to wonder why they are the size they are.
WITI syndrome (Why Is This Is).
The metel around the flash hole only needs enough lip to securly seat the primer, right? And a bigger hole would start a better fire, too?
You probably hear this one ten times a month...

[Shorty Pimp]

Don't enlarge the flash hole, higher chamber preassure could result. Buy a flash hole deburr tool if you want to clean it. They enter the case from the mouth and have a cutter head to remove the burr on the inside.

This is definitely not a necessary, procedure if you're going to be using the brass for an SKS or an AK - you'll see no appreciable gains in accuracy.

[JA]

Ditto what SP said. The flash hole in the brass is formed by a punch during mfg. this is why it is ruff looking. If you enlarge the hole it will allow more pressure to be exserted on the primer. Which could result in it failing to seal the rear of the cartridge case and letting high pressure gas leak out.
It is no fun when a primer fails,been there done this. I have a U shaped AR-15 exstractor(they are suppossed to be straight) and blowed out 30rd mag to prove it. The mag swelled to fit the mag well and cartridges,follower,spring,and floor plate were blown out the bottom of the mag.

[DeltaWolf]

Hey saed
I'm glad you asked first

I hate to see you screwup your rifle :eek:

[saed]

My second grade teacher said; "There are no stupid questions."
Thats gotta be some kind of sin to lie to a child.
Awright I wont do it! It was just a thought. And I can probably still return the tools...
...Ya, its a cool rifle. I'd hate to blow it up.
Thanks a muchomegamillion... Stu

[xIxrxoxnxwxuxlxfxxx]

What shorty said !

Pretty is not necessary to have good ignition.

HOWEVER, the Berdan style priming used in Euro countries ( and designed here ) is the strongest method, though a pain for reloading. I did it once. It just aint worth it !

All of the pressure centers in the cartridge. The bigger the Boxer style hole, the higher the pressure will be on the primer, and then the bolt face.

The proper hole size is about .063 to .064. Just over 1/16th, which is the decapping pin size. (If my memory serves correctly)

[saed]

So basicly it ends up being a pressure regulator with the compeeting goals of letting the flash out while NOT letting the pressure in...
Bigger is better for one; smaller for the other.
... After many years of use and test one sixteenth seems the best compromise for boxer?

[xIxrxoxnxwxuxlxfxxx]

As it has been about 30 years since I addressed this I went a-serfin and here is what I found:

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Precision Brass ?
Organization: Hewlett-Packard Fort Collins Site

I don't think non-flashhole cases would be worthwhile. They're pretty
good on all commercial cases; even those used for competition. Most
folks accept the flash holes as they are. Those wanting the uniformity
of primer pocket and flash hole dimensions modify what the factory makes.
Flash holes vary a few thousandths in diameter and using a small drill
bit to open them up to the same diameter is easy. The hole will be more
uniform if an existing one is enlarged rather than drilling one in a case
that doesn't have one. For example, my .308 Win. case's holes range from
079 to about .081-in. I use a number drill (44, I think) that's .082-in.
in diameter running in a low-speed drill press to uniform them. As they
are quite well centered in the first place, I don't find any slight off-
center positioning detrimental to accuracy. I then chamfer the inside
of the flash hole to remove any burrs on the inside. Primer pockets are
then uniformed for depth to about .131-in. below the case head.

In machine rest tests at long range, primer pocket depth uniformity makes
a very small improvement in accuracy, but very difficult to detect. Flash
hole diameter uniformity also makes a very small difference; again, very
difficult to detect. Some of the top highpower shooters only uniform the
pocket depth and not concern themselves with flash hole diameter uniformity.
But they do deburr the inside of the flash hole; that tends to make a more
noticable difference.

Some years ago, I did some velocity uniformity tests between cases with their
primer pockets/flashholes uniformed and other cases that weren't so uniformed.
The only thing that made any repeatable difference was pocket depth being
uniform. Flash holes varied about .002-in. in diameter; pocket depth .004-in.
All the pocket and hole uniformity accuracy improvement tests I've done
show little improvement.

I'm convinced more noticable accuracy improvement from things related to
primers is the selection of the primer itself. The primer's ignition
uniformity is crucial to accuracy. All brands vary in uniformity; within
a lot and from lot to lot. And the milder ones seem to produce better
accuracy than the stronger (hotter?) ones. Once in a while, each company
will make a very uniform lot of primers. But finding that lot is where
the time and trouble begin. I've found the most uniform large rifle primer
made on this planet is the RWS-5341. Some lots are better than others, but
I've found the odds of getting a uniform lot is much higher with RWS than
any other. In a recent test of several lots of Federal 210M primers, the
muzzle velocity standard deviation (best test of primer uniformity I know
of) ranged from 5 to 40; that's about a 15 to 120 fps velocity spread. With
RWS primers, the lots varied from 4 to 15 fps Sd; a 12 to 45 fps velocity
spread. That's about 3X the difference I've seen compared to uniforming the
primer pocket depth and flash hole diameter.

BB

[saed]

I could use a drill bit and give it spin with my fingers to deburr the hole. That'd be easy enough.
But I just measured the primer pocket depth of three lapuas and three winchesters (dial caliper)

winchester:
.129 .129 .1295
lapua:
.129 .127 .128

'Course, the lapuas are alot prettier.

[wlynn5]

Good advise all. The size of a flash hole is less important than the depth. The tool that goes down the case neck and removes the burrs around the flash hole will also give you a uniform depth if you use it befor decapping your brass. The spent primer acts as a depth stop.
Its been many years since I bought mine but is was like $5.00 then.