I am just guessing here, but I am thinking that something spinning fast enough to throw 120,000 rounds per minute at 5-8k fps just may be a little difficult for an infantryman to control...
I may not be doing the math correctly, but if you want the 1/32" projectile spacing (120krpm), this only lasts a minute or two.
Maybe the ninjas on realUltimatepower are going to use these.