Very tight actually. I have several 3 lugs TROS, Foxtrot Mike , adapters etc. The Navy style ILWT measures .587" at the muzzle which is a hair under the ID spec of .590" ( 15 MM ) for 3 lug couplers. Its larger in OD than most of other barrels and adapters by a few thousandths . The TROS being the same dimension. Honestly though I run a 45 Tirant silencer so I'm not that concerned about some slop .