a 3 foot long stable platfom capable of mounting a minigun and firing accurately? Something tells me that aim will be shunted a little.
I smell something....  
 
  
Fact is, for 
indoor you don't need a minigun do you...? Or rocket or bazooka or whatever...  

A short-barrelled, unchoked shotgun with a light (~24g) load would fill most rooms and barely needs to be aimed. I reckon a light load auto shotgun (recoil absorbed by the loading mechanism) would be easy to accommodate...  

But you could still disable it by throwing a tea towel over it... 
