Brownell's sells barrel liner drills (drill bits). I'd recommend a lathe for drilling out barrels
http://www.brownells.com/.aspx/pid=6...L-LINER-DRILLS
AR barrels are no different. Years ago I use to shoot a lot of CNGB postal matches. That's where you used M16s with sub cal devices shooting little targets (same as used in Appleseed shoots today).
I converted a Colt SP1 barrel to 22 only by drilling it out with one of Brownell's barrel liner drills and installing a 22 liner, then chambered it for 22 RF. It was quite a bit more accurate then 22s out of a 223 barrel.
It's not really that hard, the drills have a built in pilot on the front of the drill bit. Once drilled out you can use Accra gel to glue the liner in or silver solder it in. Never had one come out and I've re-lined lots of 22s besides the AR barrel. You do have to use chamber reamers and head space gages to finish the job.