Maybe a couple of things I wasn’t very clear on. When the barrel is cold and I start out shooting .22lr, I have no problem with accuracy. I can put all rounds in a 2 inch circle at 50 yards using the CMMG conversion and the barrel doesn’t get hot at all.
Just so I understand you properly, this is what I now understand to be what is happening:
1) You shoot some .22LR using the conversion kit. Accuracy is perfectly acceptable.
2) You shoot some .223, get the barrel nice and hot. It shoots fine.
3) You switch back to the conversion kit for more .22LR. Accuracy goes to pot.
What kind of barrel profile do you have? Lightweight? Government? HBAR? I'm still not sure the issue is the conversion kit since it DOES run properly to begin with.