The Magpul STR (Storage/Type Restricted) is a drop-in replacement buttstock for AR15/M4 carbines using mil-spec sized receiver extension tubes. A storage-capable version of the CTR®, the STR has improved cheek weld and two water resistant battery tubes on a compact, reinforced polymer A-frame design. A shielded operation lever and supplemental friction locking system prevent accidental operation and minimize excessive stock movement for enhanced weapon stability and accuracy.* The STR incorporates two integral sling loops, a reversible push-button QD sling mount, and a replaceable 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. FEATURES Mounts on mil-spec sized carbine receiver extension tube (not included) from manufacturers such as Colt, Armalite, S&W, LMT, and others (please see the note below) Two water-resistant battery storage tubes located on the left and right sides of the stock Each tube will store up to two (2) CR123A lithium or AA batteries Tubes are accessible from the rear in any stock position via a hand-removable rubber plug Supplemental friction lock minimizes accuracy robbing wobble* Premium chrome-silicon lock spring provides positive locking and long service life Sloping cheek weld provides a wide contact surface for user comfort Shielded release latch prevents snagging and accidental operation Replaceable 0.30" Rubber Butt-Pad offers an anti-slip surface and increases impact protection Compatible with Magpul ASAP®, ASAP® QD, and other receiver-mount sling attachments Sling Mounts Rear – Reversible Type 1 Sling Mount fits up to 1.5" push-button QD swivels (swivel not included) Rear/Bottom – 1.25" sling loops NOTE: 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 Commercial-Spec STR.