New Grendel Build Questions

[BluntForceTrauma]

Underground told me to go with the .125" as its in stock and stronger than .136"?

There are two parts to the bolt issue. Part one is the bolt body itself, with its boltface depth and the strength of the lugs. Part two is the extractor.

Technically speaking, yes, a 0.125 boltface depth is "stronger" than a 0.136 boltface depth, all else being equal. Less metal is machined away — 0.011 to be exact. All things considered, this is a very small amount.

Secondly, and more importantly, a 0.136 boltface depth allows one to keep more metal in the extractor claw. Practically speaking, this is going to affect your weapon functioning more than the difference in the boltface depths.

Grendel is designed for average pressures around 50,000 psi to account for the larger case head within the AR15 bolt constraints.

The Holy Grail is to have the best of both worlds: 0.125 boltface depth AND a claw that retains the strength of 5.56 claws.

Anyway, best to go with a 0.136 bolt, but you can go with a 0.125 bolt if your barrel is properly headspaced for that dimension and, in that case, I'd recommend having a spare extractor on hand.