Questar sells those Tactical Solutions dedicated AR .22LR uppers. There are two styles. The above M4 and a flat top bull barrel varmint rig. They are very nice.
I have found a couple of the problems with them though. I've found that the 27 round mags tend to jam. The 10 round ones seem more reliable. Also a round rubber piece fell out of mine. I'm guessing it was a buffer of some sort.
I've also had damage to two of the mags from a rimfire round rupturing inside the upper. Both times a chunk of plastic from the front of the mag blew out. One of my 10 round mags no longer works.
Lots of fun, good quality and fairly accurate. A couple of minor issues though, common to any rimfire that is being used to put large quantities of cheap bullets down range.
Here's the varmint version currently on my Stag lower.