That is a good point, 5.56. The 7x57 is a full power round while the 5.56x45 is an intermediate round.
As far as matchlocks go, I'd have to say that both the 5.56x45 and the 7x57 have several advantages over the paper cartridge that was introduced by Sweden for its matchlock muskets. The metallic cartridges of the 7x57 and 5.56 are more water resistant, less prone to damage from mishandling, are more accurate, have a flatter trajectories, are easier to load, produce less fouling and make less smoke than the paper matchlock cartridge. More importantly they have self contained primers, eliminating the need for a smoldering match.
I guess what I'm trying to get at is why do we select an intermediate cartridge over a full power cartridge? What limitations do we incur and what advantages do we reap? Are some intermediate cartridges better than others? Are there things we would like our current cartridges to do better?