This Apex 9mm Threaded Barrel is designed specifically for the FN 509 model pistols with a factory 4.00" barrel. This includes the FN 509 Midsize. The heat-treated stainless steel barrel has a black Melonite finish with contrasting stainless flutes. The barrel features a 1:10 twist rate that is designed to increase accuracy performance. The thread pattern is 1/2-28 and a thread protector is included. This barrel is a true drop-in and requires no fitting.