Over the weekend I did a thorough exam and found some interesting things.
So, maybe some of this will be interesting to the forum.
I disassembled the cartridge and discovered that the primer pocket in the Winchester casing I used matched exactly with all the other primer pockets in all the other casings in that box of 50 (.121”). This isn’t much of a surprise as I uniform all my .223 Rem. brass.
However, the casing was new unfired brass and this stuff measures out in my RCBS Precision Mic at minus .005”. This is a bit undersized.
The Remington 7 ½ BR primer also measured out as normal; cup depth .112” which is approx. what they will measure once the anvil legs are tensioned. Unseated anvil legs will be very slightly proud of the cup. An unseated primer of this type will measure out at .122”.
I think that my .009” recess primer depth was a bit too much, and the result of my being a little bit heavy handed with my priming tool….my bad.
Now things get more interesting.
I field stripped my AR and got out my firing pin gauge. Using the “maximum depth” end of my gauge against the bolt face with the firing pin pressed firmly in the bolt, I could feel no wiggle at the hammer end of the pin; so far, so good.
BUT, using the “minimum depth” end of my gauge and repeating the test, I got no wiggle again!
The firing pin was not in spec for minimum protrusion from the bolt face.
Fortunately, I keep a spare firing pin in my parts kit and repeated the test with the spare pin. This time I was able to feel a tiny bit of movement of the firing pin when I conducted a “minimum depth” test.
For those who might be interested, the rifle in question is a new build. I call it my “Cinco de Mayo” carbine because I built it on the day my town celebrated that holiday. At any rate, I installed a nickel boron coated bolt carrier group from Texas Brassworks and apparently they sent me a carrier group with an out of spec firing pin.
It seems that everything (firing pin, primer, cartridge case) was close enough that it took a combination of factors before I finally had a failure to fire with this rifle (825 rounds).
Conclusions:
1) I plead guilty to being a bit ham fisted with my priming on at least one primer
2) The firing pin was out of spec.
3) Where was the minus .005” Winchester brass case head spacing? Probably not on the shoulder, more likely on the extractor, which placed the case head and the .009” recessed primer, “Who knows where, exactly?”