Brian,
That seems to be true. The standard cartridge box that union troops carried was made to hold 40 cartridges. That is not to say that the men could not store ammo in other places like ruck sacks, etc. The question of ammo and the amount a man might fire in a fight was why the War Department as not to wild about repeating rifles in those days. I don't think they bought many Henry's for example. One reason for that was the fact that the rifle was under powered for military use and I would guess the other was becuase of the amount of ammo a man could fire in a short period. I am sure someone with more knowledge than I have will be able to answer you better than I can. Frank