Ginormous,
The barrel on my rifle is 26" long
I got the original rifle when I was a kid from an old time gunsmith. He had a stash of trapdoors he had bought from Bannerman's before they went defunct.
http://www.hudsonvalleyruins.org/yasinsac/bannerman/banold.html
Homesteaders liked the Trapdoors because they were cheap and so was surplus ammo. I think I paid $20.00 for mine.
I have seen a Bannermans catalog that listed Trapdoors for $7.50 and ammo was like 50 rounds for a quarter. They went belly up long before I knew about them.
This old guy like dinking with TD's and he had some spare barrels too.
I had run across articles and pictures of the Officer's and Markman's rifles and I just made my own cheap version.
I bought my barrel from his widow when she cleaned out his shop and closets.
I don't think Rienhart-Fajen is still in business. I stopped in their shop and got my stock blank in person. I think there are a couple of other stock makers in Warsaw MO where R-J use to be.
Bishops and Macon, I think. Maybe one of them bought out the stock patterns from R-J.
That would be a place to start.
Mine has a cheek piece like a muzzle loader on the butt and I had to fill the ramrod hole with black plastic.
Another way to go is to copy the Meacham conversions.. They would take an old Trapdoor and put a Sharp's barrel in it. I saw one of those at the Wyoming State Museum. The octagonal barre is kinda neat. A guy could get a barrel from Dixie or somewhere and stick that on a Trapdoor for something unique.
Might shoot a bit better than a Springfield barrel but mine ain't no slouch.