I noticed that when I played through the whole campaign I never seemed to pick up a Nova cannon. However, the one weapon that seemed like the one I needed to take down the Mantis (the "Buzzap" cannon?) was very similar to the weapon on the Scorpion walker. Does anyone know if the "Buzzap" is documented everywhere or has anyone ACTUALLY picked up the Nova cannon during the campaign?

I only seem to spot the Buzzap cannon (and the Nova) during the multiplayer levels.