Misfire. Short firing pin?

[Coyote Kev]

I was shooting my DPMS clone AR10 in 6 creedmoor when I had a misfire, actually several. Trigger went off, firing pin hit but no primer fire. I’m shooting new Lapua brass with CCI 450 primers and Varget. It was cold, about 7 degrees. I have heard stories about small rifle primers in the extreme cold not going bang. Is this a real thing?

This was with a Falkor high pressure bolt. Unfortunately I didn’t think of trying out my JP bolt that I have in another rifle at the time. Upon investigation several days later I found that the JP firing pin sticks out further from the bolt face than the Falkor when completely engaged. To be specific the JP sticks out 0.049 inches and the Falkor 0.043. Could this explain my misfires? 0.006 seems like a big difference in this context. Also, what is the spec? Is there a standard?

 

[Coyote Kev]

Here is the misfire

 

[EXTREMEPREJUDICE]

I have never had problems with small primers and temperature. I don't know the spec for protrusion on a DPMS style but the general rule is 1/2 the diameter of the firing pin so a .080 DPMS firing pin would have around .040 protrusion. I don't think that's your issue either. It could be several things, starting with headspace, hammer spring (backwards), retaining pin bent, short stroking, or more likely your hammer is hitting your bolt stop.

 

[MtnCreek]

Or primers not bottomed out in pocket.

 

[Coyote Kev]

Or primers not bottomed out in pocket.

I've had this happen before with the same bolt but I can't remember if it was really cold out at that time. I thought the primers not bottomed out was the problem. So I got a new hand primer. An RCBS. This one has plenty of overtravel so you can seat that primer all the way in. The hornady I was using didn't have extra travel so you got what you got. The new hand primer seemed to fix this until now. I mention the temp because I have shot this thing plenty without this problem and that's the only variable that I can tell changed.

 

[Coyote Kev]

I have never had problems with small primers and temperature. I don't know the spec for protrusion on a DPMS style but the general rule is 1/2 the diameter of the firing pin so a .080 DPMS firing pin would have around .040 protrusion. I don't think that's your issue either. It could be several things, starting with headspace, hammer spring (backwards), retaining pin bent, short stroking, or more likely your hammer is hitting your bolt stop.

It's a AR Gold fully contained trigger so it's not a backward spring.
How does the hammer hit the bolt stop?

 

[MtnCreek]

If it's a primer seated too shallow, it should fire on second strike.

New brass with very tight pockets? That can make it more difficult to tell if seated completely, for me anyway.

 

[EXTREMEPREJUDICE]

How does the hammer hit the bolt stop?

Dry fire the rifle (not just the lower, assembled rifle with magazine out), then check the movement of the bolt catch, should move easily with no resistance. The firing pin keeps the hammer off the catch. There might not be enough hammer height or proper cut out to work with your bolt or other component mismatch.

 

[AMGtuned]

Dirty BCG? I've seen the inside of one dirty enough to keep the firing pin from protruding far enough to ignite. Cleaned, reassembled, no problems.

 

[Coyote Kev]

But is there a standard for firing pin protrusion on AR10s? Keep in mind these are both “high pressure” bolts.