I bought a PSA 12” Nitride 6.5 Grendel pistol kit. Got it together Friday and shot it Friday and Saturday. Functions 100% and firing pin indentations look spot on. I decided to measure the bolts and firing pins of various firearms to see how much difference there is.
PSA 12” Nitride barrel, Nitride 6.5 Grendel BCG
Bolt length 2.808
Firing Pin length 3.275
Faxon 18” build PSA Nitride BCG Monster GB bolt
Bolt length 2.812
Firing Pin length 3.2765
Faxon 16” Build Precision Firearms Nitride BCG
Bolt length. 2.8095
Firing Pin length. 3.277
Shilen 20” build Shilen supplied bolt PSA Phosphate Prem. BCG
Bolt length 2.814
Firing Pin length 3.275
J&T Distributing upper (AA bolt) 6.5 Grendel upper
Bolt length 2.814
Firing Pin length 3.274
I also measured my 5.56/223 AR-15 bolts and firing pins
PSA 16” SS midlength Freedom
Bolt length 2.798
Firing Pin length 3.2755
RRA Build RRA BCG
Bolt length 2.7965
Firing Pin 3.279
Colt 20” Competition H-Bar
Bolt length 2.799
Firing Pin length 3.280
After looking at these different measurements I think in a mass produced industry an employee assembles a bolt carrier group from bins of parts. I suspect a gauge is used to verify the firing pin protrusion is between .028 and .036. If the firing pin protrusion is out of spec they might try a different firing pin bolt combination. If the parts fail they go in a reject bin. There is a .008 variation to account for different length firing pins or bolts. Tolerance stack and you can get excessive or inadequate firing pin protrusion.
PSA 12” Nitride barrel, Nitride 6.5 Grendel BCG
Bolt length 2.808
Firing Pin length 3.275
Faxon 18” build PSA Nitride BCG Monster GB bolt
Bolt length 2.812
Firing Pin length 3.2765
Faxon 16” Build Precision Firearms Nitride BCG
Bolt length. 2.8095
Firing Pin length. 3.277
Shilen 20” build Shilen supplied bolt PSA Phosphate Prem. BCG
Bolt length 2.814
Firing Pin length 3.275
J&T Distributing upper (AA bolt) 6.5 Grendel upper
Bolt length 2.814
Firing Pin length 3.274
I also measured my 5.56/223 AR-15 bolts and firing pins
PSA 16” SS midlength Freedom
Bolt length 2.798
Firing Pin length 3.2755
RRA Build RRA BCG
Bolt length 2.7965
Firing Pin 3.279
Colt 20” Competition H-Bar
Bolt length 2.799
Firing Pin length 3.280
After looking at these different measurements I think in a mass produced industry an employee assembles a bolt carrier group from bins of parts. I suspect a gauge is used to verify the firing pin protrusion is between .028 and .036. If the firing pin protrusion is out of spec they might try a different firing pin bolt combination. If the parts fail they go in a reject bin. There is a .008 variation to account for different length firing pins or bolts. Tolerance stack and you can get excessive or inadequate firing pin protrusion.