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