The IMI Defense Heavy-duty blocks allow you to firmly clamp M16/AR15 receiver halves in a vise without risk of crushing, twisting, or otherwise distorting them when applying vise pressure or torquing the barrel nut. Keeps your rifle secure for safe, effective cleaning, assembly, and repair. High pressure, solvent resistant, structural Rynite block and insert surround the entire upper receiver and support it inside and out to make re-barreling the M16/AR15 an easy, one-man job. Lets you tightly clamp a carry handle receiver or flattop receiver in a large bench vise for barrel removal/installation. The torque generated while tightening the barrel nut is transmitted to the block and insert, not the receiver. Minimizes the possibility of damage to the barrel detent pin and receiver detent notch. Holds securely without marring the finish.