Only works on certain bcg - FRT Triggers FRT-15L3 AR-15 Forced Reset Trigger, 3 Position, Drop-In Compatible w/Ambidextrous Safety Selector and Anti-Walk Pins

HK MR556 BCG is about 3 mm to short so won’t work with it, so put it on my DD mark18 that BCG was about 1 mm to big but was all to push the back mechanism forward to activate the trigger. So just a heads up to future buyers you might have issues with it. Don’t know if it was a manufacturing error trying to get product out due to a large amount of back orders, I know I waited 3-4 months for this.