A friend of mine is a police instructor (unarmed combat). The first time he fired a full-auto M-16 on the range, the range instructor stood behind him, hands on the back of both shoulders.
Apparently, the -16 climbs for some folks, some of them even lose their balance and tip backward, and some of those don't have the presence of mind to release the trigger when that happens. It's not universal, and maybe not common, but it's a real enough possibility that they take the precaution.
I've never fired anything full-auto, so I have no first-hand experience to bring to the table. My dad tells me that there's a range around here with a full-auto M-16 available, so I might scratch that off my list next weekend.
You're going to find that an M16 has just about zero recoil. You can fire it one handed (it will climb some if you do that).
Seriously my 13 year old (60 lbs) could fire an M16 on full auto with no problem.