Any 6.5 will do, that is the diameter for the bullet.
In general, you want muzzle devices for 7.62 for Grendel. For the most part, makers standardize on 5.56 and 7.62 sized devices.
In general, you want muzzle devices for 7.62 for Grendel. For the most part, makers standardize on 5.56 and 7.62 sized devices.

