I have a couple of M&P15-22s that are something like 10 years old. I had replaced the original mil-spec triggers with Geissele 3 Gun triggers, then last year switched in a Hiperfire 24c and an Elftmann trigger in the two guns. No problem. I used it for Steel Challenge and general practice.
Took them out recently and had a consistent malfunction. Specifically, the last round out of the magazine wouldn't fire. If I loaded two or more rounds in the mag, the first one went off but the last one didn't (empty mag at that point, obviously). Occasionally it might happen mid-magazine, but fairly reliably on the last round.
Took apart both bolts, and saw no issues with the firing pins. Swapped back the Geissele triggers, but no joy. Finally swapped back one of the original triggers in one gun, and it immediately began to run correctly.
What I don't understand is what could have changed over the years to cause the failures when the same configuration worked before. Obviously, I'd like to put back the better triggers so I can use them for competitions.
Any ideas?