I'm pretty sure I know the answer, but I found out today that "aftermarket parts" won't fit my Colt LE6940. Apparently, everything is proprietary on a Colt Ar15? For Christmas I got four 42-round promag magazines and a cool ejection port cover (says "let it rain!!") The magazines jam every other shot and the cover won't install (needless to say, i don't think the cover CAN be installed...this port cover is unlike anything i've seen in online tutorials of how to change port covers) I'm pretty sure the reason is simple - if you change up the gun, they want it to always be "Colt"....just wish I had read that part of it before deciding on this gun...might have gone with Rock River instead. Though I guess it's not all bad, i'll just have to find some Colt-specific aftermarket parts...anyone have any recommendations?