This chamber groove cutter was designed to, shockingly, cut a groove in the bottom of your chamber to aid in BB centering. Only the smallest of grooves is necessary, and in fact desirable, as making it any larger than is necessary for centering will only cause blowby. Centering improves chambering consistency thus spin axis consistency. To use this model, abrasives must be bonded to the long narrow panel (highlighted in black in one of the images). This can be thin strips of crocus cloth, sandpaper, used sanding belts, or lapping abrasives. (long-lasting abrasives are preferred) The cutter may then be aligned with the barrel, and partially inserted. The jig, which hold's the cutter's orientation relative to the barrel, may then be slid over the assembly. The back of the cutter will fit a 1/8" square drive rod, the same rod used for lapping, which may be bonded to it. The cutter must then be pushed and pulled until the desired cut has been made. Repeating this with finer and finer abrasives can result in extremely high surface finishes.