So how much of the work does the eventual owner have to do on the firearm? If you want to register a kit car as a custom vehicle you have to have put a certain number of hours into it, as I recall, or maybe a percentage of the total hours.
The part that gets the serial number from the manufacturer is considered the firearm. In the case of an AR style rifle this would be the lower receiver, For a 1911 handgun this would be the frame. As to how much work has to be done