The Fixed Mount is designed for carbine/subgun platforms that have fixed barrels. The Fixed Mount is a stand alone mounting accessory that replaces the booster system in the Obsidian 45/9. The Fixed Mount is not used with the 3 Lug Mount or Fixed Barrel Spacer. Available Thread Pitch Options:1/2x281/2x365/8x24M18x1M13.5x1LH.578x2811/16x249/16x24 Rugged Suppressors .578x28 Fixed Mount OF007