The Sinclair Vise Block is the perfect tool for securely holding your AR-15 in a bench vise. The Vise Block inserts into your magazine well just like a magazine and the bottom half of the block can be clamped into a vise. You can tilt the block in the vise to work at a different angle if desired. You can also insert the vise block into the top of the lower assembly allowing you to work on the bottom side. The Vise Block is made of machined high density polyethylene, so it is extremely solid. Great workbench tool.