The Wilson Combat TRTTUM2 Tactical Trigger Unit (TTU) is an exceptional single-stage trigger unit that you can easily install in any AR pattern rifle within one minute. Its precise engineering offers a robust, 4 lb. ultra-crisp trigger pull without needing any fine-tuning or adjustments. A shortened positive trigger reset assures a swift follow-up shot on target.Its unique hammer geometry maximizes dependable ignition with a variety of military-grade and surplus ammunition. Mirroring the 1911 style, the TTU features a half-cock notch, ensuring it meets or exceeds drop safety military specifications. Its trigger housing-pre-assembled and CNC-machined-contains an H13 heat-treated trigger, hammer, and sear, along with a stainless steel spring for effortless installation.Designed with incredible attention to detail, and rigorously tested by shooters for shooters, the Tactical Trigger Unit has revolutionized expectations for drop-in AR triggers. The Wilson Combat AR-15 Tactical Trigger Units are guaranteed to work flawlessly with any standard Usgi style AR style lower receiver that has standard GI trigger pins of .154\" diameter, and is caliber-neutral, including 7.62/.308 length AR receivers.Please note that there are some compatibility exceptions: Colt brand rifles manufactured for commercial use between 1991 and 2009-usually having a trigger pin size of .169\"-and Colt rifles produced with pinned-in sear blocks are not compatible with the Wilson TTU. This trigger unit is also not compatible with Bushmaster ACR. The MR556 and any other AR style rifle equipped with a firing pin safety mechanism in the bolt carrier can exclusively employ the TTU 3G (TR-TTU-3G) or Two Stage Models (TR-TTU-M2).In terms of .22 LR AR conversions or full-blown .22LR caliber AR-pattern rifles, we advise using the standard single-stage TTU (TR-TTU) or two-stage TTU (TR-TTU-M2) trigger, preferably with high or hyper-velocity ammunition for top-level reliability. It should also be noted that while the TTU is compatible with the S&W MP-10 .308 AR, the rifle\'s ambidextrous bolt catch must be removed prior to trigger unit installation.