The Obsidian Arms upper receiver vice block is machined from High-density polyethylene (HDPE) to hold your upper receiver in a bench vise without fear of crushing, marking or scratching the finish. Simply secure the assembly into your vice, slide the upper onto the block, and insert the provided locking pins. Features: Fits all standard AR-15 upper receivers High-density polyethylene (HDPE) Locking pins included Mfg: Obsidian Arms