Flak is the only new one currently available.
It looks like there are 3 other weapons in the works but currently not available...
- Railgun: Ultra long range S/M gun which cannot turn, so locked onto facing of ship.
- Some sort of charging gun weapon. Possibly like the one from XR.
- Cannon: Data points towards this being a gun form of Flak.
Toughest and most heavily armed M freighter. Has reasonable speed as well, only beaten by Paranid freighters.
If you can find a use for such a freighter currently I do not know. Supplying stations in hostile space maybe? Maybe with the up coming pirate mechanics it will make more sense. Until then it seems better to use conventional freighters and have them drop cargo to pirates rather than duel them.