Looks like a great water-pistol, or foam-dart gun, IMHO. But I could see how depending on the material it could seem much more "real", as well.
I like what EvilSOB picked up on, but for the sake of originality I'm going to guess that the big barrel is actually full of smaller rotating barrels (like a minigun), and the pump is pulled to "detonate" all barrels at once, at the cost of some more recoil and a delay until the shooting continues.
And it's for a platformer/shooter hybrid with four-armed characters. You can fit any one arm with a single-handed weapon, or any two arms can share a two-handed weapon

Jibb