New FFL here. (Well, second time around vs. 25 years ago)
A customer wants to buy a Muzzleoader with 209 ignition. Is the 4473 required? is shipping to an FFL required? It is not a model of Muzzleoader that has interchangeable barrels that would enable firing with a modern cartridge/ammunition ability. There's conflicting information out there.
Shipping to New mexico.

If it's not capable of firing modern ammunition then I would think it does not require 4473 or FFL. 

https://www.atf.gov/file/56421/download

I didn't know either. There are exceptions...you should read at the link.

Thanks- A local large shop here keeps them in a separate logbook and no 4473 is needed according to them, they have about a 2k inventory.

A muzzle-loader is considered an "antique firearm" (even if it was made yesterday) and is exempt from the definition of a "firearm."  Therefore, no 4473, no logging it into your A&D book, etc.  

Now, be careful... some newer "muzzle-loaders" are made on the receiver of a firearm (i.e. Remington 700 Muzzle-loader).  Even though the "muzzle-loader" is not a firearm, you are still transferring a Remington 700 receiver - which is a firearm!

Happy to send my credentials for this answer. :)