Ramshackle Games has some more options for their quad-cannon turret set with some different weapons you can buy for it.
From the release:
Some new pieces have just gone up in the Ramshackle Games web store. These are based around the Anti Air quad cannon. It is available in the same way as the cannon from a few weeks ago with a trailer. With the trailer its priced at £15 or just on its own at £8. The Anti Air gun can also be had with the Gnu at a deal price of £18.