STI Recoil Master. Reduces Recoil - Plain and Simple. The recoil master is designed for one purpose - to reduce felt recoil. It decelerates the slide before full stroke with a secondary spring. This allows a softer felt recoil helping you to shoot faster follow-up shots. The light system is designed for 9 mm and open pistols. The heavy system works well with 40 SW and 45 ACP. Fits STI 1911 and HiCap. Must be able to remove the guide rod from the rear of the slide. 5.0" Light, Bull. Note: STI Recoil Masters can only be used in STI Pistols.