I haven't shot a full auto rifle, but i've shot the M16A4 on burst. Even at 25yds, the shots open up quite a bit. its definetly tight enough to keep it in a torso but its not very precise. I can see the need for them in close quarters, but other than that i wouldn't ever put it on burst.
Its fun as hell to shoot though. The most fun I've ever had shooting was on the 240 and the SAW