There are 4 different sizes that are possibilities: .625", .750", .875", and .936". I've never seen a .308 AR barrel that uses the .625" gas block diameter - that would be one thin-walled barrel! :o Most common are .750" and .936", on standard and bull barrels, respectively. .875" isn't "uncommon," but it's just not as common as the other two sizes. You don't need to be precise in the measuring to determine what yours is - it's pretty basic. About 3/4" in diameter (.750), 7/8" (.875), or 15/16" (for the .936).