I'm considering the CMMG 22LR conversion for AR-15, but I have a question: what happens to the gas that enters the gas tube?
The gas, carbon and lube (on lead bullets) are a non-issue if you clean it, and or shoot a few .223 rounds through it periodically. There is less gas and carbon, even if the lube burns, than from .223 ammo.
But, if you have a bullet that happens to lead the bore, or you have an old barrel with a worn gas port, yep, you can (not will) create some severe issues.
I've seen some gas tubes with large bulges in them, blown primers and very erratic accuracy from some conversion guns where the barrel was let to be very fouled, and or the gas port was pretty eroded. Usually, it is just some poor accuracy for 10 to 15 rounds while the lead gets blown out.
Copper washed bullets can (not will) mitigate the issue.
Personally, with a .22LR BCG AND barrel being about the same price as a .22 Conversion kit, to me, just build the upper and avoid the potential issues.