Here's the Athletics skill, with some example stunts. That would seem to feed the Musketeer style you're going for. Are there any Musketeers in Louis family or friendships? Note the the novels were set from roughly 1625 to Louis' present.
Muskets are, pretty much by definition, smooth bore with loose fitting projectiles at a low speed. The loose fit was to handle powder residue as multiple shots would happen between chances to stop and clean the bore. Precision shooting is possible, but very rare and it requires a lot of focus and attention.
On a related note, I'm not above taking liberties with physics, 1600's steam-punk, and history, if we can make a great game of it.