The Promag AR-15 Lower Receiver Vise Block holds AR-15 and M16 lower receivers steady on the bench while you install or swap parts. It works as a third hand, so the receiver stays put while you drive pins, seat springs or fit a new trigger group. Clamp the block in your bench vise, then slide the lower onto it through the magazine well. The magazine catch engages the block just like a magazine and holds the receiver in place. The block is injection-molded black polymer, so it grips the receiver without the metal-on-metal contact of clamping the lower directly in the vise jaws. Features: Fits AR-15 / M16 lower receivers Inserts into the magazine well and is retained by the magazine catch Clamps into a standard bench vise Injection-molded black polymer Made in USA Typical jobs include lower parts kit installs, trigger and hammer swaps, pistol grip changes, mag catch and bolt catch work, and buffer tube installs. Before you start: confirm the magazine catch is fully engaged on the block before you put force on the receiver. Snug the vise only enough to hold the block. You do not need to crush the polymer to keep the lower steady.