1. From what I've seen most percussion rifles were octagon. I think it was easier to make than round barrels.
2. They made .32 and .36 caliber rifles for small game hunting.
3. I doubt it, most stuff today that's water proof isn't. They were probably water resistant though if made right.
4. They used pure lead and you should still use pure lead in BP guns. Wheel weights are a alloy and shouldn't be used for BP guns as they are too hard.
5. I don't care for modern muzzle loaders. I prefer old school using granulated powder, patches and ball or lead bullets. I have some newer sabbots and powder pellets but havent gotten around to use them. Might never use them either...