I'm not sure how much you want to differentiate between early musket armies vs late musket armies... I'm sure an 1860s musket army could defeat a 1600s musket Army, but a musket is a musket as far as most Civ players think, eh?
However, in Civ terms I always consider a "Rifleman" to properly be a late 1800s army (1870s and beyond)... roughly a Franco-Prussian to Boer War era army and I think the uniforms of Civ units should match that period.
That is pretty much my thinking. The uniforms of the riflemen though seem to be a cross of the musket and rifle..
I just hate seeing the Musketmen or the Rifleman representing the Musket Infantry. One has it's name wrong, the other looks wrong. But to most it is satisfactory I guess.
For differentiating this is how I actually tried, but didn't get enough time to play much.
Musketmen are the same.
Musket Infantry come with Replaceable parts (After which they could be used as the sole or at least majority of the infantry).
12 Pwr, 1 move, %25 vs cavalry, whatever cost.
Rifleman Comes with Rifling and some other tech that would be a major part in the breechloader concept.
16 Pwr, 1 move, %25 vs Cavalry, %10 vs gunpowder units (So on par with a combat 4 Musket Infantry)
Grenadier, Chemistry, 10 pwr, %50 vs Musket Infantry, Upgrades to Rifleman
The thing is, the Matchlock musket was about as similar to a Brown Bess as a Brown Bess to a Rifle. (They were much heavier) Plus Matchlocks were far from the only weapon used in a formation, I would wager that pikemen were used as much if not more. The Rifle is important today, but the rifleman in game went from (As you say) 1870-1900 approximately. Where as the Brown Bess type was used from 1730's-1870's, 140-30 years. And it wasn't like it was all peace time either (Seven Years War, the War of the Austrian Succession, the Napoleonic Wars and not to mention the American Revolution).
I just tend to think that even though it is Beyond the sword, they forgot the musket. At least they added Curriassers though..
You just look at the variation in the classical age, axeman, spearman, swordsman, archer, catapult, and in the Early Industrial you got the Cavalry, Riflemen and Cannon.. I mean if they are willing to differentiate between a longbow and a crossbow, or even a spear and a pike, you think they would give difference to a late and early musket.
And By the way, I did start the helmet for the Colonial guy, looks quite good if I say so myself..