I haven't tried it myself, but I've heard that you can clog up your gas tube using a .22 conversion kit in an AR. Because of the coatings on most .22 ammo and the fact that it usually burns dirtier than 5.56 ammo, lead and carbon fouling will end up in your gas tube. Supposedly if you fire a few 5.56 rounds after every hundred or so .22 rounds it will blow out the gas tube, but even if that works the lead and carbon fouling is still going to dirty up the rest of your gun faster than with 5.56 ammo.
Lead and carbon fouling aside, the rifling twist is going to hurt the accuracy of the smaller .22 rounds, and you're going to have to re-adjust your sights every time you switch back and forth. Many people save themselves all the trouble and just get a dedicated .22 upper.
0331: "Accuracy by volume."