But that doesn't make any sense. In that case, a NAC/10 has 1/5 ammo/ton, or 0.2 rounds per ton, while a NAC/20 has 2/5 ammo/ton, or 0.4 rounds per ton. The ammunition gets lighter with increasing weapon size?!
If, however, the NAC/10 is 5/1 ammo/ton, or 5 rounds per ton, and the NAC/20 is 5/2 ammo/ton, or 2.5 rounds per ton, the ammunition is getting heavier with increasing weapon size. That seems a bit more intuitive.
But if that second progression is the case, and therefore the fraction is 5/1 ammo/ton, then sub-capital cannon ammo is much, much less efficient than full capital cannon ammo.
EDIT: Ah, thanks Cray. Makes sense. Kinda sucks for the sub-capital systems, but makes sense.