.22lr conversions in .223/5.56mm barrels is not accurate enough for hunting, in most cases. I assume that you mean small game like rabbits and squirrels. The .22lr bullet is a true .22 inch diameter. AR bullets are .224 in diameter. barrels for the AR are wider than for .22lr, thus .22lr bullets do not "grip" very well in the larger barrel. Rifling twist rates are also much faster in the AR barrels and the .22lr bullets don't stabilize well. Results are lost velocity, erratic accuracy (12" groups at 50 yards in some cases), heavier leading in your barrel, etc. That being said, inside 30 to 20 yards, you may be close enough that the above doesn't matter that much and you can still get your squirrel. This is where your stalking and other hunting skill come in.

The .22lr kits really shine for cheaper training options. If you want to hunt, but maintain your AR system, I recommend a dedicated .22lr upper that aleviates most of the above problems.