Alternatively, you could use the game designer's fiat, and state the parameters you want your gun to have, give it a really cool name, and bingo, there it is!
Just remember that:
a) if you slaughter their characters with your new shiny, your players will not want to be in a game run by you again.
b) if by a miracle they survive, they will use that gun with glee against all your plot mooks, so be prepared to be slaughtered right back again. If it uses highly specialised ammo and 'just happens' to have run out the moment its previous owner croaked it, refer back to a) above.
