What is the general timeline of each of the eras for the story folders?

The descriptions are a little vague. Medieval warfare says pre-gun but gun powder weapons have existed in Europe since the 14th century. Modern warfare can encompass a lot, are we talking the modern warfare of World War 1 or of the present day, like in CoD4?

I'm asking this because the story I want to add will have a magical version of the flintlock musket. Medieval weapons will be used alongside the rifle units, but I'm not sure if this will qualify as medieval or modern warfare.

I think and I could be wrong in that matter,
but the key point the main weapon of the war.

So medieval is based on melee weapons, like swords and armor-like chain mails.
Where Gunpowder is more relevant is the Gunpowder Age, which needs to divide into 3 times-ears,
-up to Napoleon, with front-loader musket and fighting in lines,
-up to the equal of WW1 the trench warfare,
-WW2 and the wars after, where the tactics and weapons similar to your are developed

WW1 and WW2 saw a basic shift in the conduction of war to be based around open ranks warfare and machine guns. while they existed before WW1 this is when they fully became the standard rather then the exception. WW2 saw a true combined arms taken to the modern understanding with mobility and weapons. In the early time of muskets and muzzle loaders fit into the idea of older warfare that one would see with large closed rank units, but you see the start of open rank units becoming more common. If you have lots that can fit into pre WW1 stile weapons (black power, Muzzle loading line combat/ old west feel) then i would make a new folder if its not that many they just make one folder.