Re: Jonathan Browning Moutain Rifle BN Value
you can probably go to a book store pick up a gun digest, a paper back looks like the size of an old sears & robuck catalog.
this book lists most everything made & sold in the U.S. and would list the current prices.
the rifle has a good reputation & following. i have seen the regular rifles sell from 400-800 on the auction sights.
you could check feed back from historical trekking, & frontierfolks web sites , also traditionamuzzleloaders.com may give you some idea on worth.
in your case a collector sounds the best bet for top dollar sale.