Burned through 500 in about 8 seconds a few times...
I just did that today. Went to the range just to do it, and on the way I bought a Winchester 555 bulk pack of .22. Threw the empty box in the trash before I went home, my Mark II is pretty filthy now.
You must have lots of Mk II mags
Me shooting an American 180. It's 170 rounds of .22lr every time you pull the trigger. I went through three 550 round boxes of .22lr that day loading for me and other folks this and three other .22lrs:
It's fun but even more fun in a single shot or 5 shot 22 rifle plunking varmits in the garden or barns. Make your shots count and oh... maybe not as impressive as firing as fast as you can pull the trigger but... in a day you will be hitting what you aim at. 22 shorts were 28 cents a box, longs a bit more and long rife were 37 cents a box as a kid. Still at the time it was expensive for a kid. (man in a factory making 90 cents a hour) Learned that about 100 yards or so to fire 6 inches higher then the blackbirds head and shoot. I'd fire pull the gun down watch a puff of feather the bird fall and hear the plunk. It taught me sneaking up on crows was one hard job, but great fun the a kid.
M134 Vulcan?
M134 Minigun at 4000rpm. Been lucky enough to shoot them a few times...without having to buy the ammo...