Your rifle is an original midwest gun, based on its architecture. Without a barrel signature or initials behind the rear sight, it will be difficult to determine who made the rifle, but the lock plate was apparently sold by a hardware store in Madison, Indiana, owned by "G. W. Leonard & Son." There was a George W. Leonard in Madison, Indiana in the 1830s through the 1850s. He was not a gunsmith, but rather served as a trustee in the Christ Episcopal church in Madison for many years (which showed some status in the community), and later formed a bank in Madison in 1841 when railroads were being constructed through Madison. He was a local businessman, and could well have had a hardware store there in the 1850s (although I have not documented it) when your gun was probably made.
A fellow Hoosier, Jeff Jaeger, is perhaps the most current and informed researcher on Indiana guns, and he is (slowly) putting together a book on the subject. I will contact him and ask him to review your post, and see if he can add anything about G. W. Leonard, or the rifle itself. Shelby Gallien