I have  a few questions about bolt/firing pin specs.

What I have found for specs.

Firing pin protrusion––––––––-    Min.  .028         Max.        .036

Firing pin hole in bolt––––––––    .058  to .060

Firing pin tip dia. ––––––––––––     .054

I am assuming these are correct.


I pierced some primers with the upper on the first trip out and cratered the firing pin so I ordered two more firing pins... replaced the cratered pin today.

What I found:

Firing pin hole in bolt is .065.
Firing pin tip dia. on all three firing pins is .060 to .061.
Firing pin protrusion on all three firing pins is .038 to .040.

It seems both the bolt and all of the firing pins are "out of spec"...... but the clearances or fit are OK..... .004 to .006 difference between the firing pin hole dia. in the bolt and tip of firing pin.

I am wondering "IF"  the manufacturer/builder got some out of spec bolts and had some out of spec firing pins made up to match the bolts??  Again, I am going by the specs I have found.

I expect the rifle to function fine ( no more pierced primers) as the clearances are OK and I chucked the replacement firing pin up in a small lathe and filed and stoned the radius on the firing pin until the protrusion was an in spec. .034.

So, the question is, if I ever have to replace the firing  pin on this upper, and I order a pin from another builder etc. and I get an in spec. firing pin, I'll have trouble.

Are the specs I have correct??   And, are the firing pins I have as bad as I think??

What say you??

Is this an AR15 chambered in .223, or does it have a 5.56 chamber?

Next:  What ammunition were you using?

If .223 chamber with 5.56 ammunition, pressures can jump into areas that will show these signs.  With prolonged use as such, you could experience worse.

Need more details about the rifle, ammo, and chamber dimensions.  Lots of companies mark their lowers .223 Remington, but the chambers are 5.56.  

A 5.56 chamber has a longer leade, and can take projectiles with more tangential ogives, loaded with hotter military propellants to achieve the original design goals of the Small Caliber, High-Velocity rifle concept, with an ability to perforate steel helmets at 500yds.  5.56 is a very hot load when assembled to US specs, and other nations that have rifles that can actually take the pressure, unlike the L85 and FAMAS, for examples.

Thanks!!

5.56 chamber..... Federal XM193 and Federal Fusion pierced ... 2 of each out of 20 of each fired.
PMC XTAC and Hornady Match did not pierce.

Are the firing pin hole dia. in bolt  and firing pin specs different between a .223 chambered AR and one chambered in 5.56??

Since they use the same bolts, I can't see there being a difference.  I can imagine a firing pin hole to the larger or slightly over the edge of spec, plus hot XM193 producing some craters, and or piercings.  Maybe the Federal is over-charged as well.

I actually deprimed a bunch of PMC recently, and all my primers were GTG.  I just got a bunch of the Federal XM193 in from PSA, so I'll see what it does.  The best bet is to chrono it and see what velocities you're getting.  If it's sailing past 3150fps from a 16" carbine, it's probably over-pressured.  Also, if there is a ridiculous extreme spread in the cratered or pierced primer cases from the others, that would be an indicator.

Could also be excessive headspace, but since you don't see the signs in the other ammo, I tend to not think so.  Excessive headspace would allow more primer expulsion to the rear during the burn, and a larger firing pin hole would make craters, but since this also does not happen with the other ammo, I would chalk it up to hot Federal XM193 if I was a betting man.

^^^^ Thanks.

Just wanted to be sure if I had to use another firing pin sometime, that the replacement would be similar in size.
I adjusted the new firing pin to protrude .034 so I should be fine... it was .041.
Rifle shot fine and I didn't realize the primers were pierced until I checked the brass.... other than the few pierced primers, no other problems.

What is the range in diameters of the tip of firing pins??