Its a little easter egg I stumbled upon while fooling around on the new map.
How to Get It
To get the gun you have to break into the Vault while looking like a Bank robber (The easiest way to get the outfit is to get the level mastery to 13 so that you can spawn as a bank robber in the garage right next to the vault, otherwise there is a couple in a bathroom right off the entrance in the bank locked in a stall, there is a bank robber outfit in there with them, then you have to find a way down to the vault in the outfit) I dont know if you can turn off the lasers in the vault for this because i just walked around them to get to the core.
You can walk past the lasers pretty easily since they only have a simple pattern.The last thing you have to do is pick up all the gold bars in the vault then the lasers turn off ,when you turn around there should be a pile of gold in the middle off the room with the shotgun sitting on top off it. The shotgun shoots gold confetti as an effect (it still shoots real bullets though) and the bunny mask on the outfit also turns gold. (The gun is only available on this map, meaning you cant spawn with it).