The one in the top left is only 500 USD, are my eyes deceiving me or is this a replica, it says you need to a drill a hole in the barrel, so I think it starts as a replica but can be functional.
You probably dont understand what a firearm needs to be to be declared a firearm.
you have to machine-drill your own things and such, thats why you can order the parts to make a M-16 through the internet and get it thru the mail, because until you drill everything, its not deemed a technical firearm by the US government, which means you can get firearms without serial numbers by making them yourself, but just having the parts delivered to you.
This would be the same thing, just with a flintlock rifle rather than an m-16.
im assuming you have a family member that has tools, probably a drill press and such