Interpreted to the letter of the law, fully automatic means it fires continually so long as the trigger is depressed (as in blowback, delayed blow back, or gas operated type actions). I always found that interesting. There is nothing in the law about the caliber that I've seen. A 22 rimfire that is true full auto is as illegal to own without a license as a 9mm or a 50BMG.
One other thing I noticed, and found VERY interesting. A BAR cannot be transferred to an individual, according to the regulations I've seen. I've seen several for sale, and they are all listed as "no transfers to individuals, regardless of license". Guess the BATFE finds the old BAR truly frightening.