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 CTRA(R), 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. Made in U.S.A. FEATURESMounts on mil-spec sized carbine receiver extension tube (not included)Sloping cheek weld provides a wide contact surface for user comfortTwo water-resistant battery storage tubes located on the left and right sides of the stockStorage compartment in tail of stock with positive latching access door on right hand sideEach tube will store up to three CR123 lithium or two AA batteriesTubes are accessible from the rear via a tethered, hand-removable rubber plugSupplemental friction lock minimizes accuracy robbing wobble*Shielded release latch prevents snagging and accidental operationPremium chrome-silicon lock spring provides positive locking and long service lifeReplaceable 0.30" Rubber Butt-Pad offers an anti-slip surface and increases impact protectionCompatible with Magpul ASAPA(R), ASAPA(R) QD, and other receiver-mount sling attachmentsSling MountsMid - Included reversible Sling Mount Kit - TYPE 3 fits up to 1.5" push-button swivels (swivel not included)Rear/Bottom - 1.00" sling loopNOTE: There are two basic sizes of M4 Carbine receiver extensions (buffer tubes): mil-spec and commercial-spec. If you have a commercial-spec extension on your rifle, make sure to order the Commerical-Spec ACS.