Primarily designed for running .22lr with your obsidian suppressor, Our Fixed Barrel Spacer allows more versatility when using the Rugged Obsidian. The Fixed Barrel Spacer replaces the spring in the Obsidian and is used with the Rugged Suppressors Piston for use with fixed barrel hosts such as carbines and subguns. The Fixed Barrel Spacer is not used with a 3 Lug Mount or Fixed Mount.