So I mentioned that I fired my CMMG rimfire kit for my AR while I was at CCA, right?
I had a couple of failure-to-feed issues, say four or five in fifty rounds, most likely due to the fact that I neglected to lube the conversion kit before dropping it in the upper, which caused it to be rough as a cob, and the springs that run those things are pretty feeble.
More exciting is the fact that rimfire conversions on the AR are straight blowback rather than locked-breech, and that they will fire when out of battery by a fairly significant amount. With some of today's .22LR ammo running higher chamber pressures than .45ACP +P, stuff can be expelled from the ejection port rather forcefully when this happens.
In my case, a round of Aguila Interceptor fired with the bolt not fully closed. I was fine, the gun and conversion kit were fine, and I was mostly just glad I was shooting right-handed. The kit has since been sprayed with Strike Hold and cycles a lot more smoothly; I'll be shooting it more extensively this weekend.