I don't specifically remember the rate of fire but probable 5-6 rounds per minute.
The video I watched looked like they could do four a minute (I timed it) maybe five, but I couldn’t see how they could speed it up at all. Looked like they were humping it as it was, but...if things were crazy, I could see how people could really bypass some of the safety things and squeeze out an extra round.
Ugh. It looked to me, though, that the things they do are somewhat written in blood, so they might have to be really hard pressed in combat to ignore them...