How is that legal 
It's legal, it's just restricted to hell and back. Rifles like that are collectors items and strictly regulated by the ATF. Also, rounds for those are incredibly rare. I'd rather not have one of those myself because, frankly, I'm not rich enough to be able to shoot it.
Guns today can easily blow someone's head off from 50 feet, back then the best musket might be able to hit a target at 10.
Mmm, yes and no. I'll preface this by saying I'm just nosing in here because I enjoy sharing information about an interest of mine.
Your typical musket used in the American Revolution, such as the Brown Bess (used by the British) or the Charleville (used by the Americans) were smooth bore and would be used with under-sized rounds which made them inaccurate, but they still had an effective range of between 50 yards (150 feet) and 100 yards (300 feet). However, the Americans made use of the Longrifle (better known as the Pennsylvania/Kentucky Longrifle) which made use of an elongated barrel of around 4 feet in length as well as being one of the first rifles to commonly use rifling. Their effective range (depending on the experience of the user) was between 100 yards (minimum) to 200+ yards (600 feet). To put this in a little perspective, the military issue back-up iron sight on my AR-15 has a minimum distance setting of 200 meters (656 feet). 50 feet is about the mid to upper range of a typical handgun.
