I had a Colt 9mm AR several years ago. Fun gun, but I needed the cash and sold it for a handsome profit. Conversions are available as are complete rifles. The 5.56 versions are the lions share of the market so that is most of what is currently made.
As far as caliber, how about 9mm Win Mag? I do not know of any ammo currently available, but Starline makes the brass. Could be interesting, but the blowback, unlocked breech system of the 9mm Luger AR's would have to be beefed up for the higher energy Win Mag.
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"Robo is right" Fuzzball