If, however, you modify the gun to shoot modern cartridge ammunition (eg, installing a cartridge conversion cylinder), then you cannot sell the gun with that modification installed without a federal excise tax license. You may, however, remove the modification, sell the gun as a percussion revolver and then sell the modification separately without the tax license.
Not...so...sure.... of this.
As long as you are manufacturing for private use (for yourself), and not "engaged in manufacture" as an enterprise, the occasional sale of self-manufactured firearms was legal the last time I checked. It's the intent at the time of manufacture that comes into play: Make it with the intent to sell it immediately and you need a license. Make it with the intent to keep it, and then later change your mind and wanrt to sell it... that's fine. It's exactly the same "soft" definition that is the issue when guys rent tables at gunshows to "sell their private collections" as opposed to getting a FFL and renting a table to "do business". Like ****, I can't define it but I know what it is when I see it, and that's the same thing when the Feds look at gunshow "provate sales" and "selling homebuilt firearms".
They are *firearms* once they are permanently modified, and can swing back and forth between being "firearms" and "antique firearms" if all they have is a conversion cylinder. When you add a conversion cylinder you are the "manufacturer" and you can un-make it later.
But: there is no permanent prohibition against sale of a personally manufactured firearm as long as you're not making them for sale to begin with. One of the differences is that you can have non-serialized hombuilt stuff "at home" but apparently you need to add a serial number and manufacturers name to what you might want to later sell.
A Walker is a .45-60 BTW ...