Optimal Barrel Length for XCR 6.5 Grendel pistol?

[MeatAxe]

Depends on whether the barrel is chambered to use .125” bolt face depth or .136” bolt face depth. As long as the 7.62 and 6.5 Grendel are machined to accept the same depth bolt face they are the same. Original Colt 7.62x39 AR-15 used a .136” bolt face depth to get more metal in the extractor. Most of the AR-15 market went with .125” bolt face depth with the 7.62x39. The 5.56/223 AR-15 bolt has a .125” bolt face so was not changed for some 7.62x39 AR-15 barrels.

The AK chambered for 6.5 Grendel use the same bolt as 7.62x39 AK’s.

Sounds like they went with the MK 1 (.125") bolt pattern instead of the more common MK2 (.136")

Well, you’ve got me. The Rob Arms parts catalog for the bolt says something like “7.62x39 Type 2 (Note: same as 6.5 Gren).” Of course, the XCR has a more AK-like 3 big lug bolt compared to the AR Type smaller multi-lug bolt.