I just have one thing to say about your sentence: "But I think they can still be effective against human enemy. Human don't respawn and not everyone can afford magic armor."
It might be true that, based on what you say, guns with "normal" bullets will be effective against bandits and all, but if the other countries know that Central have a "Musket squad", they could create squad with magic armor to counter it.
And sorry in advance for my poor english x)