These units replace the magazine floor plate and internal floor plate on the 9mm (13 round only) and 40 S&W. They will add approximately 1" inch in length and additional capacity.(2 rounds to 9mm and 1 round to the 40 cal.)*Check your state and local laws regarding magazine capacity restrictions.These magazine extensions incorporate a similar texture patter to that of the factory grip.NOTE: These extensions will not add capacity to the 10 round 9mm magazines and will not work on the XDM frames.