Bufford,
I have both a buckmark & a Challenger III. I have noticed that several, but not all, parts are interchangeable between these two. I haven't seen many parts listed for the challenger II. However, i found several links and forum requests for what you are asking. here's a few links and they have a 5 1/4" barrel mentioned:
http://rimfireshooting.com/index.php?showtopic=2160
http://wap.armorama.com/modules.php?...d=25157&page=1
and the browning site:
http://www.browning.com/customerserv...lete/index.asp