The Magpul ACS (Adaptable Carbine/Storage) - Mil-Spec Model is a drop-in replacement buttstock for AR15/M4 carbines using mil-spec sized receiver extension tubes. Dual, rear accessible, water-resistant battery storage tubes and an integral storage compartment in the butt of the stock provides multiple storage options. Similar to the Magpul CTR, the ACS also features an anti-snag release latch to prevent accidental activation along with a supplemental friction lock system that minimizes excessive stock movement for enhanced weapon stability and accuracy. The ACS comes standard with a 0.30 thick rubber butt-pad which provides positive shoulder purchase to prevent slippage even with body armor or modular gear."