I find the ammo-clip impractical too. It's a way too small. So small the mecha even may have problems reloading with that. Anyways it's great enough to be saved to my comp.
What's making us assume that it's a solid-ammo canon? If it were a directed-energy weapon, the physical size of the clip wouldn't matter anything like as much; with a high enough charge capacity in those packs, he could be carrying upwards of a hundred shots or some odd.