I think that is the part he is not getting. The barrel will have to be removed from the gun, chucked in a lathe, the muzzle turned down and threaded. You may be able to find a suppressor with the same finish and diameter so it will look like a LOONG bull barrel. Shouldn't call it a bull barrel then. INHO a bull barrel is very heavy and shorter than normal. A heavy barrel is heavy and as long or longer than normal.
A 24" heavy barrel on an AR is a heavy barrel not a bull barrel.