Well that's the thing, I dont even know what it uses for ammo. This is what the author says:
Description:
This Mod Places an downscaled Tyranid Impaler Cannon in the Abandoned Tent.
The weap do not use ammo since it grows the bone spikes inside the weap, but since your character is not a Tyranid, there are only limited Bio-mass reserves to do it (low durability), which you can replenish bu feeding it (repair with) Bio-mass. Theres also a Pyrovore (google it!) Larva which you can feed organic matter to get bio-mass.
As the weapon is living, breathing thing it sometimes has things to "say" (not anything comprehensible thou)
Oh, and one IMPORTANT notice, sine both the larva and the cannon are living things, and fairly hostile
to anything non-Tyranid:
DO NOT TRY TO USE THEM WITHOUT AN INTERFACE SYMBIOTE, OR THEY WILL TAKE YOUR HAND OFF.
Can be repaired w. Bio-Mass
Ignores weap resistance an pin limbs (like rail rifle). Automatic but about 180 round per minute fire rate, and kind of high dmg.
Uses custom:
-scope
-sounds
-projectile
As I said, other than getting the mesh and texture in the game I can't work out what else to do.