The TTU is a single-stage trigger unit that is easily user installed in any AR pattern rifle in less than 1 minute. Wilson Combat engineering enables a durable, 4 lb., ultra-crisp trigger pull without any fine tuning or adjustment. Reduced, positive trigger reset guarantees a quick follow up shot on target. The unique hammer geometry enhances reliable ignition with military and surplus ammunition of all types while a 1911 style half-cock notch allows the TTU to meet or exceed military specifications for drop safety. Pre-assembled, CNC machined trigger housing contains H13 heat-treated trigger, hammer, sear and a stainless steel spring for trouble-free installation. Bullet Proof designed and extensively tested by shooters for shooters, the Tactical Trigger Unit has raised the bar for drop-in AR triggers. Wilson Combat AR-15 Tactical Trigger units will reliably function in any standard USGI style AR style lower receiver with standard GI trigger pins of .154" diameter regardless of caliber, including 7.62/.308 length AR receivers. Colt brand rifles made for commercial use between 1991 and 2009 will generally have a proprietary trigger pin size of .169" and are not compatible with any Wilson TTU. Colt rifles produced with pinned-in sear blocks are not compatible with any Wilson TTU. Wilson Combat TTU is not compatible with the Bushmaster ACR. The MR556 and any other AR style rifle with a firing pin safety mechanism in the bolt carrier is only compatible with the TTU 3G (TR-TTU-3G)or Two Stage Models (TR-TTU-M2). With .22 LR AR conversions or complete .22LR caliber AR-pattern rifles it is recommended that the standard single stage TTU (TR-TTU) or two stage TTU (TR-TTU-M2) trigger be used along with high or hyper-velocity ammunition for best reliability. The TTU is compatible with the S&W MP-10 .308 AR but the rifle's ambidextrous bolt catch must first be removed to install the trigger unit.