This is a Magpul Industries Stock known as the MOE Carbine Stock, model MAG400-GRY.
The MOE Stock - Mil-Spec Model (Magpul Original Equipment) is a drop-in replacement buttstock for AR15/M16 carbines using mil-spec sized receiver extension tubes. Designed for light" fast action the streamlined A-frame profile avoids snagging and shields the release latch to prevent accidental activation. The Mil-Spec Model includes a standard 0.30" thick rubber butt-pad which provides positive shoulder purchase to prevent slippage even with body armor or modular gear.
Features:
Replaceable 0.30 Rubber Butt-Pad;Compatible with Magpul ASAP®, ASAP® QD, and other receiver-mount sling attachments