I've been told you can mount most .308 surpressors on smaller caliber guns, and not give up much performance. I'm looking at mounting a .308 Phantom on a .223 rifle.
Thread cut might be a problem.
I think the thread pattern is different on different caliber suppressors to prevent someone from putting, say, a .223 suppressor on a .308 by accident.
If you are having the barrel custom cut it shouldn't matter, but if you want to take off the flash suppressor on an AR and thread on a .308 suppressor you might be out of luck.