What does this look like to you?

It looks like a 1077 with an AR military stock, a Crosman AIR 17 front sight and upper handle assembly.  I has a modified bulk feed tank attached. 

how did they hook up that bulk feed? Is it drilled through the stock?

CO2 cartridge in the tube.

Looks more like a shotgun to me. Only reason it looks similar to an assault rifle is because of the carrying handle which is above the trigger and the protuding sight. Its frame is too small to look like an AR, which is larger in the middle and around the receiver. The 1077 looks like the Benelli M1 Super 90. It also looks like a Remington 11-87 Police/Tactical shotgun or other similar looking shotguns.

Here's a link to some that resemble it.
http://world.guns.ru/shotgun/sh24-e.htm

either a Rem 870 or Mossburg were used to make. looks like a simple mod. Go natchez shooters supply web site and you can find the parts you need.

Art

a 10/77 with bulk setup and stock,forend, handel assy, and front sight from an air 17. Looks like can dancing fun to me .