Let me start by saying WOW. I am having very similar issues w/ both of my QC10/RMW ARs, one in .45 & one in 10mm. It gives me hope that w/ this info you guys are sharing I may get them to run reliably. I have had slam fires;  I have had one of them go brrrup for 3 or 4 rounds; constant failure to feeds, failure to extract, failure to eject?; my mags seem "loose" in the magwell....yea problems. I oiled them up (like an Irishman goin to the beach),cleaned the bolts,  changed mags, changed ammo, changed shooters & even had friends who are Colt armors look them over & try to run them (aka a smart person unlike me) to no avail. I spoke w/ QC10 who gave me some ideas but no luck. I emailed RMW & well that did not go well "it ran for me" 
So they both sit in my safe waiting for rust or the gov't to take them away. (I live behind enemy lines in the republic of new jersey)
I'm going to try the ideas you folks have shared & maybe sacrifice a chicken to the gun-gods & see what happens.

This is my first post, my name is Don, I am as mechanically inclined as Ray Charles.