I've researched Kentucky's early gunmakers for about 40 years, and the information I was able to find on the Stephens gunsmiths, father William, son William Jr., and another unnamed son, can be found in "Kentucky Gunmakers 1775-1900" Vol.II Biographies. A "William Stephens" signed rifle is illustrated in the just released Vol. III of "Kentucky Gunmakers." It was probably by the father, since it does not have "Jr." after the name. Your rifle has a long, two screw tang like the Stephens rifle illustrated, and a few other possible Kentucky details [guard's front post, position of nose cap out beyond the rear ramrod pipe], but very few other similarities. I wish we knew the name of the "other" son who was a Stephens gunsmith, but I have not been able to find him. However, the father and two sons worked together from their arrival until the Civil War forced their shop to close, so all their rifles [not many are known] probably look rather similar. The sons reopened the shop for almost 20 years after the war, but those years were too late to make a gun like this. I wish this were a KY product, and perhaps there is a slight possibility, but it doesn't have much in common with the known William Stephens rifle.
Post Script: I just did a quick census search for 1860 and found that the unknown son was John Stephens. The census gave the following information: William Sr. 62 and born England; William S. (Jr.) 33 and born England; John 36 and born England. Unfortunately, we don't have a son with the first initial "B."
Shelby Gallien