My pitch on this whole thing is simply that weapons that require loose powder to be muzzle loaded are considered primitive, as are paper-wrapped cartridges (as in American Civil War rifles). This would include matchlocks, flintlocks and so on.
Those that fire brass cartidges are considered modern, be they breechloaders, Winchesters, Springfields or what have you. These all could/would/should be included in Marksman.
Gatlings would be a Heavy Weapon at about late TL 4. Maxims and Lewis guns are Heavy Weapons at about mid-TL 5. Gatlings, and Nordenfelt type pieces could be considered Primitive Heavy Weapons. Maxim guns and later are just Heavy Weapons using the Gunnery skill.
Cannon and mortars would not change too much in that TL range because their actions didn't change. While a Parrot rifle of the American Civil War might shoot farther and do more damage than a falconne of the English Civil War, the basic functions to lay, load, and fire the guns didn't change all that much. Thus they could all be bunched into the Primitive Artillery Feat.
Insofar as earlier gear goes (ballistae, trebuchets, etc.), there could just be a simple Early Siege Engine feat that implies expertise with artillery that does not use gunpowder.
Now, my big question is this... WHY are we spending time on feats that no character in their right mind would purchase???