It doesn't fire "one million rounds a minute." Not only would the magazine have to be huge to hold one million rounds of any rifle ammo but .22 rimfire - but firing that much ammo that fast would melt the barrel!
I posted an article on this several months back. 1m/minute is the theoretical rate, not the sustained rate. A MAC 9 has a rate of 900 rpm, but a 30 round magazine. If you're interested in the details run a Google search on Metal Storm, I recall there were still photos and maybe a video of test firings.
Regards
J.R.