This is something I'd like to elaborate in character, but this is actually his second hand canon. He made his first before he could use his Aura and it broke his arm. After he developed his Aura and made the dampening weave he made a new hand canon with fire dust to be even more powerful. In terms of how big a Grimm it can kill I'd say a single shot for most if not all Feind class and some in Riesen class. Kinda crazy now that I think of it, but it's his answer to tough armor and hide. He won't be using it often given the ammo limit though.

As for you second question, did Coco ever remove her shades in a battle?