NX-17 charge rifle: only requires vehicle power, but variable effect can endanger you/the vehicle (explosive!)
Flamethrower (milspec model): Drains gasoline, short range, explosive effect can damage the vehicle
Fusion rifle: Good damage, requires both vehicle power and fusion cells (manual-loaded!)
M249: Comparatively-common ammo, needs manual loading, inaccurate at long range, pretty vehicle-safe
M2 Browning: Fairly rare ammo, needs manual loading, really inaccurate at long range, likewise vehicle-safe
mk 40 GL: 40mm ain’t cheap, wide variety of ammo allows custom effects, at range is effective despite inaccuracy
That’s all the ones I know about. Blaze is theoretically working on a turret package, but I’m not sure if we’re gonna get a PR or if so when.