There was a Martin Luther Rizer back in Cumberland, MD, but this rifle looks too late to be his work, and the signature does not match [not even close] his signature. The barrel signature is actually a little odd, in that it is not centered in the barrel flat, the first two initials are much closer together than normally found on a longrifle barrel, and the size of the letters is rather large. However, since we don't know who made the rifle, perhaps this is a good signature.
The rifle looks like about an 1850 rifle with German silver inlays; I have not seen those particular inlays on a rifle before, so it's a little hard to place your gun in a specific region. Dating is based on the capbox style, heavy crescent in the butt plate, and less height in the butt. Not a lot of help, but perhaps someone else will recognize something in this rifle and can tell you more.
Shelby Gallien