Pierced primers

[AR-10]

I've got an Armalite AR-10 that is giving me a small problem. It's chambered for 7.62X51.

The firing pin leaves a very deep depression in primers. I've fired about 600 rounds through it using FNM (Portugese), Hirtenberger, and Venezualan. Of the 200 rounds of Hirtenberger I've shot, I found three casings with a pierced primer. The Hirtenberger uses a softer primer, apparently, as it is made of different material, and I have found no spent casings of other brands that have been pierced.

it has been suggested that I check the firing pin protrusion, and switch ammo.

Her's my question. The trigger is rather stiff, as a result of the tension put on it by the spring that drives it forward. Would changing this spring cure both problems? A lighter spring, if they make one, would make the trigger nicer. Would it also cause the firing pin to impact the primers with less force? Could I possibly just take the spring out and over-flex it a bit?

Sorry this is so long, Thanks.

[Art Eatman]

Hmmm. Lemme see if I understand you.

"The trigger is rather stiff, as a result of the tension put on it by the spring that drives it forward." "it" is the trigger, and not the firing pin?

Anyhow, I don't think a lighter trigger pull make any difference with your primer problem. A lesser firing-pin spring won't make the trigger pull notably lighter, although it might lessen theprimer problem. What a lighter firing-pin spring will do is slow your lock time, which is undesireable.

Again: Check the pin protrusion. If it's okay, change ammo.

Art

[AR-10]

Art,

Sorry about the poorly worded description at the beginning of my post.

The spring I am referring to is the hammer spring. It drives the hammer, which drives the firing pin. It also influences how many pounds of pull it takes for the trigger to release the hammer.

I don't know how to measure the firing pin protrusion. What tool is used to do this?

I suppose one fix for the problem would be a slightly stiffer rebound spring on the firing pin, but I'm not an expert on this stuff.

I was just wondering if altering the hammer spring would benefit both the trigger pull and lessen the force of the firing pin.

One more question for you. What is lock time? Please forgive my ignorance.

[bad4u]

Lock time is the time elapsed between releasing the sear (the hammer begins forward) and ignition in the cartridge. For a given gun design, 'lock time' can be affected primarily by;
1) Weight of moving parts (hammer and firing pin).
2) Hammer spring.

My experiences with punctured primer cups have been with tournament shotguns, which typically may fire 1000 rounds or more per week during the shooting season. Most have been due to firing pins that have been embrittled due to many firings, resulting in pieces of the firing pin tips breaking off near the tip of the pin - producing a 'sharpened' firing pin.

Have you examined the imprint left by the firing pin on the cup of the primer? The imprint on the primer cup should be fully-round. The shape of the firing pin tip itself should be fully-round.

The combination of a pierced primer and a hot cartridge can result in damage to both gun and shooter. The only damage I've personally witnessed was a ruined buttstock pistol grip and a bleeding hand, but I've heard of much worse damage.

I recommend resolving the problem by taking the gun to a gunsmith.