I'm sorry but it reminds me a spray or water gun :P
But you said it has only 1 shot so I don't have a clue of what it could be... here are some more tries:
Spiderweb gun (lol)
Oil gun (makis evriting shlipperry!!)
Dematerializer gun (Jesus Christ what a name)
Dang, something that can pass through that storage to the gun itself... it's hard to think because the connector is too thin...
Zapchon
.... bizon?
Kenamy
noe