UM TACTICAL Delrin Shim 1.0 Degree for Vortex Venom & Viper, Burris Fastfire II, III, Docters and Sightmark Optics Delrin Shims are made to fit between the Burris Fastfire red dot and the mount. Fits; Burris Fastfire II, III, Docter Red Dot Sight & Insight MRDS