UK-spec barrels with muzzle threads are M18x1.5 (metric) As of winter 2010/2011, AI-North America (AINA) is shipping rifles with barrels from a US manufacturer. These barrels are threaded 3/4-24 (US standard). This confuses things somewhat. You can tell the difference because 3/4-24 threads have a pitch of approx 0.042". M18x1.5 threads have a pitch of approx 0.059". If you aren't sure, check with your AI dealer or give us a call.