Or other parts in the upper? I'm referring to conversion kits and such. Thinking about a dedicated upper too....

Rifles are different.

I repeat that statement a lot.

I have shot .22's through at least four of my AR15's with no problems except changes in point of impact on the target.

However, my son's AR15 leaded the barrel very badly with Federal .22 ammo (his favorite).  This was "jacketed" (which is probably plated) bulk pack ammo purchased from WalMart a few years back.  It took several clearing sessions, multiple barrel brushes, a small mountain of patches and finally some lead remover patches bought from Midway to clean the lead out of that barrel.  It's a chrome lined barrel but it must be awful rough inside to shred up the .22 bullets like that.

Rifles are different.  What works fine in mine might not work in yours.

Try yours and see.

Any issues with 1:7 barrels?

I assume the CMMG uppers are better than rhea S&W 22?

.22 LR wants a bore diameter of .219", while your AR's bore is more like .221" so those soft lead bullets will probably engage the rifling only with the skirt at the back of the bullet.  That's not a "problem," just a factor to consider.

Copper plated .22 bullets don't have wax lubricant on them, so the wax can't possibly build up, though that's not a major problem with most barrels.  On the other hand, you could get lead build up at the gas port - which could take anything from "thorough cleaning" to "run several full-power rounds through it" to disassembling the gas system to remedy.  As noted, it "depends," and usually what it depends on is unpredictable.  The best way (in my opinion) to deal with this is to inspect your barrel carefully after every .22LR outing, and to pay attention to how it runs when you're shooting centerfire ammo.

Build up in a .22 barrel is hard to predict. Barrels vary. Rougher ones can pull a little lead off of the bullet as it goes by and you can get some very bad lead build up. Of course that also varies with what ammo you are using.
A couple of years back I left a box of .22 on the table at the range for the next guy. It had leaded up my barrels so badly I didn't want to ever let it go through any of my .22 firearms again. (It left some serious lead in three of my firearms).

Anybody tells you to never clean the barrel on a .22 is full of chit. I have seen some pretty bad stuff happen with .22's due to ammo/barrel condition. I have had issues with both dedicated and conversion rifles so you just don't know. As I said, there are many factors at play here so its hard to say which firearm or ammo combination is going to cause a problem. Therefore....check those barrels! Don't buy into that stuff about cleaning your .22 wears out the barrel. Maybe it would if you really did it badly or abused it but normal cleaning won't hurt it. I have some firearms that have had tens of thousands of rounds in them and are still in great shape. I don't want to give up safety and accuracy on my firearm by NOT cleaning it and having it foul.

My experience is like that of forever4.  I had dedicated .22 rifle that would shoot well for 25-50 rounds from a clean barrel but precision would degrade as lead built up.  A quick pass with a bronze brush dipped in CLP would restore performance for another magazine or so.  Then I would have to repeat the treatment.  I tried lapping the barrel to smooth it but success was limited; I could go two magazines before cleaning.  I finally sold that barrel, bought a CMMG barrel and never looked back. I still push a wet patch through the new barrel and clean the bolt after each range session but rarely see any lead slivers.

In the past, when I used a 22LR conversion with non-dedicated uppers, I did get an occasional failure to cycle when switching back to centerfire ammo after a brick or more of 22.  I think once it took three rounds to clear the gas system and get the rifle cycling with 5.56 again.  Usually, however, the very first centerfire round after shooting 22 would cycle just fine - guess it depended somewhat on the 22 ammo.

I pretty much only shoot dedicated uppers with 22 now, though.

My Melonite CMMG 4.5" doesn't ever get build up. I would assume that the meloniting helps because it's so slick.

That could be true. Or, you could just have a very well made barrel with a nice smooth finish.  Or it could be both of those. Or it could be the ammo you are using. Or it could be all three!

Not trying to be funny, just state the obvious. I do know that I, like others, have had better luck with name brand, good quality dedicated .22 uppers especially when using good ammo.