A 20 round burst would bring the muzzle so far off target it was crazy, but the muzzle cage for 16's is designed to reduce flash, not reduce the effects of recoil which drove the military to make the 3 round option which you well know the history of. My comments for general history.That whole 3-5 second burst never went well..... M-16 would rise to the right every time!!
A properly clocked muzzle brake on full auto can put every round in a small circle which would also result in everybody in the area becoming deaf. I've seen pics of some of the SF's using brakes. I hope they have some superior hearing protection.
I RO'd during an indoor match once where the shooter had a Rolling Thunder Muzzle brake installed. Even with plugs and muffs over them, my ears rang for the rest of the night. Horribly loud.