The patent-pending Magpul® CTR™ (Compact / Type Restricted – Commercial Model) is a drop-in replacement butt stock for carbines using commercial 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 extra-power lock mechanism uses a premium chrome-silicon spring to avert auto-collapse failure and maintains strength by resisting spring relaxation. The CTR™ also features a supplemental friction lock system that eliminates wobble* for enhanced weapon stability and accuracy. The Commercial Model includes an extended 0.55” rubber butt-pad to provide positive shoulder purchase and accommodates longer slant-back extension tubes. Note: There are two basic types of M4 Carbine Receiver Extensions (buffer tubes): Mil-Spec and Commercial. If you have a Mil-Spec extension on your rifle, make sure to order the Military CTR™ (MAG310). For help identifying which extension is on your rifle, click here.