Hi,
If you Google Henry Leman, you will see that he was a very prolific gun maker.
He produced flint plains rifles up to at least 1870, well into the cartridge era.
Many were in large caliber, .58 and larger.
Leman ( Lehman ) produced thousands of rifles for the western movement, and very little credit is given for his great efforts.
So you will not be too far out to make a very late flint plains rifle.
Good luck on your venture.
I have just finished a .58 flint plains rifle, 1/2 stock, and am starting a matching pistol and rifle plains gun in .69 cal.
Fred