The master key was a good idea. Basically just remove the stock of an 870 ( the receiver to stock hardware accepts a bracket made for the M4)
It's really cool in video games. The reality of the situation for those of us who know what breeching a structure is like is this:
Anywhere you would find a use for a master key, an 870 would be ideal anyway. An M4 is a fine cq weapon but the more crap you attach to the more cumbersome it gets. Now you are carrying a designedlight rifle, with a heavy shotgun receiver in a very poor position. Reaching the trigger is a pita, and now instead of holding the rifle typically, you are clutching the fore-end of a shotgun much lower. Extra ammo is heavy.
Personally, for forced entry where there are known assailants, 870 Police Magnum is my number one choice. I'll save the black rifle for a more open environment.