[Kylehman84]

Something I noticed today is the firing pin seems to be contacting the primer when chambering. Pictured is an unshot round which shows an obvious pin indent. My question is could this lead to an unwanted, accidental discharge? This is my first AR and I (Gunsmith) recently added a Geissele SDE trigger…not sure if that has anything to do with it? I’m a centerfire guy so this has me a bit alarmed.
Thanks for your help!

 

[FidiusAchets]

How long have you had the rifle? Or more accurately how much have you shot the rifle?
Are all of the bolt carrier components stock and are they in tolerance?
How dirty is the rifle?
Can you provide a better picture of a spent casing next to an chambered but unfired round?
Is the primer strike excessively deep on a spent casing?

Is this just the first round chambered out of the magazine or any round that you stop firing on.

To me this looks like the firing pin may be bent or there is trash in the bolt carrier somewhere causing the firing pin to not move freely during chambering. The firing pin may also be out of spec. Perhaps not in overall length but the position of the lands that contact the retaining pin in the bolt.
It may also be the case that the pin is moving very freely and this mark is just that, a mark from the pin slamming into the primer when the bolt carrier comes to a stop.
My current AR is in 7.62x39 so I predominantly shoot Berdan primers which I have measured to sit lower in the case head than Boxer primed amo of the same caliber. I discovered this while chasing down light strike and chambering issues on this build.
I would advise disassembling the bolt carrier group completely clean and reassemble. Fire some rounds and re-examine. I am sure there is someone else here that has more insight to this.

 

[Doom]

The AR has a floating firing pin Every round chambered is going to have a slight firing pin indention since the inertia of the pin causes it to contact the primer as the round/bolt chamber and the pin continues forward. This is why it is recommended to use military grade #41 primers, OR magnum primers which have harder cups.

 

[Harper]

That is normal with an AR. Free floating pin taps the primer as the bolt closes. It will not fire. Or if it ever does, then something needs doing, of course (pin too long or jammed in forward position, or some such thing). But it would be an anomoly. I use regular primers (not magnum) and if I were to look at a successively chambered round after firing, I would expect to find this dimpled primer every single time. I have never had one fire as a result.

 

[300Boomer]

Where the shoulder of the brass round seats when chambered ,t there could be brass build up from a lot of previous firings. If so , then round would be pushed a little towards firing pin spacing causing hit. Just a thought. In Nam, I carried a CAR- 15 on recon and do recall hits but not always. To me if you see this allot it said,s clean me.

 

[Doom]

300Boomer, thanks for your service.

The M1/M14 firing pins are prevented from moving toward the cartridge until the very last bit of travel of the bolt. When both bolt and firing pin forward motion have slowed, the bolt rotates the firing pin into a receiver recess to allow forward movement of the firing pin. This method was used to reduce the potential of a slam fire or firing out of battery. They did not typically dimple a primer. The ARs/M16 was designed with no stop or spring (as in the 1911) to resist or reduce the forward inertia of the firing pin. When the BCG slams forward the firing pin is forced to the rear. As the bolt and cartridge seat and the bolt rotates to engage the lugs the firing pin travels forward to strike the primer as the bolt carrier stops. The firing pin doesn’t have enough energy to compress the cup and anvil but does have enough energy to dimple mil-spec primers which are the recommended primer for 556 to prevent slam fires. Out of battery protection is provided by the design of the bolt carrier which prevents the hammer from striking the firing pin until the bolt has been fully rotated by the bolt carrier.

The only ARs that I have seen that do not dimple the primer on every round are ones where the firing pin was bent or dragging in the bolt. They typically exhibit failure to fire.