Trigger pull did it without question.
Installed a Brownells AR15 reduced power spring set (Yellows), and had it polished, cost me $35 total.
Dumped three mags, with sufficient cool down in between, no issue at all.
Only warning the smith had was that the lighter strikes could misfire on some of the cheap foreign ammo because of harder primers, so just for grins I bought 40 rounds of tulla steel to test that theory...
Sure enough 1 misfire, dimpled but not ignited.
M855 and M193 blew through it like a breeze.
( Could not video, was at range by myself, could not hold it and phone at the same time
Now to clean the tulla poop out of my gun, man that stuff is like nasty dirty...