Helps counteract the painful effects of recoil using mercury, one of the heaviest, densest, liquid elements. Compact, permanently sealed, steel case is made from bar stock; the only moving part is the liquid mercury so there's no wear, no chance of leaks. Mercury's viscosity varies only 1% from 0 F. to 90 F. which means the recoil-fighting performance is consistent in almost all weather conditions. Stock models fit in the rifle or shotgun stocks and are drilled and tapped 1/4"-28 tpi for easy installation and removal.