I do not think there is any legislation in the United States surrounding Blackpowder rifles at all.
NO.
Investarms of Italy
At least since the 13th century
You need to consult with a lawyer for a correct and current answer.
I am sure the first rifles made using black powder did not have a serial number engraved. Antique rifles without a serial number do exist. I am however unsure what the law state about collector pieces. Logically one can not force a serial number onto a piece that originally was made without one.
To make reloading faster and more accurate.
I imagine state laws vary on this but in Texas you do not have to call anybody to transfer a black powder gun.
no, only caps for percussions pistols and caps for percussion rifles. gun stores know the difference.
Assuming you are of legal age to purchase, black powder or smokeless powder for reloading can be purchased from gun shops. Yes, proof of age is required.
Sure. I like pictures of Hawken rifles.
Depends on where you live, and WHICH .50 cal. California has banned rifles that fire the .50 Browning Machine Gun cartridge. Other state have not. Please note there are several .50 cal firearms- including .50 caliber muzzleloading black powder rifles.
check with your state Attorney General Normally no, though there may be exceptions concerning hunting rifles. A petition to the state (a pardon) may also be worth looking into. Contact a lawyer for professional legal advice.