Just put in my order for the Guncrafter Industries .50 GI conversion kit for the G21. It consists of a new slide, barrel, and 9 round mag. Sights on the slide are standard Glock fixed sights. No mods required for the conversion. They also use a 22 lb. recoil spring, IIRC. The gentleman I talked with said that the Sprinco recoil reduction systems should fit the .50 barrels just fine. Kits should be shipping in about 3 months. First orders in will be first ones shipped out. $599 for the kit.