Heavy-duty blocks allow you to firmly clamp AR-15/M16 receiver halves in a vise without risk of crushing, twisting, or otherwise distorting them when applying vise pressure or torqueing the barrel nut. Keeps your rifle secure for safe, effective cleaning, assembly, and repair. Unbreakable urethane vise block takes all the clamping force, so your expensive lower receiver won’t be damaged or distorted. Insert block into the magazine housing from the top or bottom, then clamp the other end in your bench vise. The magazine catch locks to the block, safely and securely holding the rifle for assembly, disassembly, cleaning, or repair. Will not mar or scratch the finish. Fits tightly in most receivers and may require filing to fit others.