The Mk 1 Apex Grade Semi Drop-In 4.25" Barrel is designed for the 9mm Smith & Wesson M&P. Due to the tolerance variations in the M&P Slide, the barrel is semi drop-in. In some cases, this means the barrel may drop-in without alteration, but in most cases will likely require minor fitting which is easily done by the end-user. The stainless steel barrel features a 1:10 twist rate. The design increases accuracy performance making the M&P capable of shooting as small as a 1-1/2 inch grouping at 25 yards.