I have had what appears to be primer cratering on one of my rifles.  It occurs on all but the very lightest of loads and appears consistent up through the heaviest loads that I have fired.

One friend suggested that the firing pin hole might be over-sized.  This opinion was reiterated in Glen Zediker's handloading book.  Zediker wrote that the hole should be 0.058" to 0.060".  Another source told me that the maximum allowable was 0.0645".  The only gauge that I had available was a 1/16" bit (0.0625") and it passed through the hole with plenty of extra clearance.

Does anyone have some experience with this that they could share?

Normally, primer cratering is associated with excessive pressure... I'm assuming that you're handloading?  Have you checked the headspace on the rifle?  Post a bit more info on the rifle, ammo, number of rounds, etc...

The bolt in question is a Les Baer chrome-plated.  The barrel is an Olympic Arms Stainless Ultra Match.  This has resulted in a very tight chamber.  The fired cases still fit into my case gauge - I can get by with neck-sizing only, if I choose to do so.

The "cratering" appears the same whether the load is 24.0 grains of Varget behind a 69 grain bullet or 24.5 grains of Varget behind an 80 grain.  The respective amount of primer flattening varies from none visible to well squared corners.  The amount of case swell between a full-length sized case and a fired case is 0.001" regardless of the load.

I really don't think that it is a matter of being over-pressure.

I would assume that very tight chamber also correlates to higher chamber pressures. You are shooting heavy match bullets with long ogives. I would also suggest there is not much jump space from the neck of the cartridge to the lands of the rifling. I think you just have pressure plain and simple. Some powders I know nothing of Varget really jump in pressure due to temp. changes. My AR10 with AA2230 will shoot fine on cool day and blow primers on a hot day.

Back off your load. Accuracy is not dependant upon everything being a max load.

Thanks for the input.  I just wanted to follow-up on my original post.

1)  I found out the the maximum specification diameter of the firing hole is 0.0645" and that is where mine is at.

2) I swapped e-mails with Glen Zediker ("The Competitive AR-15" on the subject.  His advice is to avoid anything over 0.060" or you will have a "cookie-cutter" effect.  

Looks like I'm in the market for a new bolt.

Right on the money, I ran Olys CNC Milling center that made their bolts, .058 min-.063 max.



Great advice, but-


How can you make sure the one you get will be at that, its not easy to measure that hole. Calipers are usaully used but do not give a true measurement, only a CMM will take an exact measurement. add .001 to .0015 for caliper measurement. Make sure if it is checked that its not being measured on a burr on the hole.
I would ask Glen Zediker if he has a source for bolts with this size hole.

Fritze Out

Soft primers??

Undersize firing pin nose??

I have been using Winchester Small Rifle primers.  My understanding is that these are on the hard end of the primer metal spectrum and suitable for semi-auto use.  Is this valid?



I do not have a way to measure this or have specs.  Three firing pins have been used with the same effect.  They appear the same as any other mil-spec pin that I have seen to the eyeball.

smooth_pilot:

I just threw out those two ideas as something to think about before you went off and bought a new bolt.  Granted, the bolt sounds like your problem.

I always like to start with the cheap fixes first and then work my way to the expensive ones!!  [:D]

I don’t know about the hardness of Winchester primers.  (I do know that someone – maybe CCI – makes a hard so-called mil-spec primer for use in AR’s, but that’s to protect against the floating firing pin during chambering.)

Still, you might’ve gotten a weak batch of primers.

I’d be tempted to run a few commercial loads through that AR to see what happens.  (Obviously mil-spec loads would probably really jack up your pressures in that tight chamber.)

If you haven’t, I’d also run some of your reloaded rounds through another AR.

Since you already tried swapping out firing pins, that idea’s pretty much out.

Just brainstorming, of course.  You’re the guy on the ground!

Good luck!!

199,

I will run some of this ammunition through my Bushy.  It needs some exercise anyway - don't think that I have fired it since I started shooting High Power.

A buddy of mine has some CCI primers - I think.  Probably has some commercial ammo, too - but he might not admit to it.

Thanks for the ideas.

Try using the bolt from another rifle, see if the problem Persists prior to buying a new one.