I have to admit this always had kind of bugged me as well, but we are talking about a Sci fy video game. You'll notice when reloading hand canons that the cylinder that is replaced does not contain bullets so I chalk it up to the idea that nearly all the guns are a type of energy weapon and we are simply swapping batteries when we reload. This also makes synthesis make sense as it could be just a charge pack that our ghost uses to rapid charge cells. LMGs use belt fed single use rapid discharge cells, that happen to look like bullets, for high dmg output. However this is just how I justify the mechanics in my own twisted mind and is not based on any lore.