Gunplummer,
I referred to it as “Rule of thumb” cause calling such a simple dimension a “formula” has always seemed silly to me. The dimension was taught to me and everyone else in my class in the first semester of college by the instructor in charge of the gunsmithing program I attended. I have always adjusted pin protrusion in this manner and have never had a firearm fail to discharge. This is how the protrusion was adjusted as well when I worked at McMillan Rifle Co.
You guys got me curious so I mic’ed four AR firing pins, 1 ea. measured .059 dia., 2 ea. measured .062 and 1 ea. measured .063. The .062 were DPMS and the other 2 were from unknown manufacturers. I also used a gauge pin to check the firing pin hole in the bolt face of a Bushmaster carrier assy. and that came in at .070, firing pin protrusion was @ .030 on that unit. I had never checked the diameters of pins for an AR before cause it is never an issue with this type of rifle….if a pin breaks, then just throw in another one. They don’t have to be adjusted during assy. as some bolt guns do.
I imagine (in regards to military gauges) that the max (Go) and the minimum (No Go) protrusion on an AR (M4,M-16,etc.) would be to insure that the military never had any misfires cause by too little firing pin protrusion. On a firearm chambered in a non-magnum cartridge….a little too much protrusion wouldn’t cause any undue problems as long as the tip has the correct radius, and I’ve only seen maybe three firearms that had to have the pin reshaped in order to keep them from piercing primers. Being that military ammo can also utilize slightly harder primers, the added length (Go-gauge .036) to the protrusion would also help ensure ignition if the headspace of a particular weapon were at max to begin with.
I have never owned a military pin protrusion gauge cause all it will tell you is if the protrusion passes or fails and that is determined visually, I use an adjustable gauge that allows me to check the dimension with a depth mic…….no guessing involved as to how much of the pin is sticking out of the bolt face. I have had to use the military gauges once before though, the Navy supplied them to us when we were building the .50’s for them and wanted the pins set using their gauges, this method did make setting the protrusion of the pins much faster.
I agree with you that “if it ain’t broke, don’t fix it”, all you end up doing is spending more time in the shop working on crap when you could be spending that time on the range. The OP was simply asking what was the optimal protrusion for an AR, I gave him the “Rule of Thumb”
that I was taught. This particular question is kind of subjective though when dealing with military style weapons. He never said he was going to alter the pin and some of the replies to this thread kind of beat him over the head.
I'd be more concerned to check the tip during cleaning to make sure it wasn't damaged.