Here's some good general info on the plane. Boomerang
As you can find pretty easily searching on the internet, the armament was 2x 20mm Hispano cannon and 4x .303 Brownings MG's. (with the MG's mounted in pairs outboard of the cannon on each wing)
What I haven't seen is the ammo capacity.
Also I think the CAC CA-13 was thw Mk.II, the CAC CA-12 being the Mk.I Boomerang.
The CA-13 was the Mk II with 95 being built. I have the ammo capacity as 1000 rounds per gun for the 4x .303 MG's and 60 rounds per gun fror the 2x 20mm cannon. Bomb load was 4x 60lb smoke bombs carried under the centre section.
Wiki has this drawing showing the belly tank.