Does it use the 5.56 barrel and upper? All you change is the mags and b/c?
Yup. The 10 round magazines are actually an insert that goes into a standard 5.56 mag--20 or 30 rounder -- either one.
The replacement bolt is just a very short barreled "zip-gun" that just fits the chamber and guides the .22 until it reaches the throat of the regular barrel.
You don't have to mess with the buffer and spring, as that doesn't even get used. To charge the .22 adapter for firing, you only pull back the charging handle an an inch or two and the spring built into the adapter is actually what you are compressing.
I use copper plated .22LR, cleaner than solid lead. When I am through playing with .22, I replace the standard bolt&carrier and shoot a half dozen rounds of ordinary 5.56 to blow any gunk out of the gas tube., and then just clean as usual.
These kits are lots of fun! I like them for shooting steel targets in the pistol range (no center fire rifle because it just blows right through the plate) and squirrel hunting.