If you have a .54 why do you want a .50 its only .040 smaller in bore diameter, there is not a sufficient difference in balistics to consider. It's your choice of course but I can't help wondering why?
A fast-twist 50 would be a good choice - along with aperture sights - check IdahoRon's long-range conical renegade - has a green mountain 1:28" (I think) barrel that shoots conicals into 1" at 100 yards. Now, how cool is that?
Check with Fox Ridge.[url]
http://www.foxridgeoutfitters.com/[/url]
Or keep an eye on the auctions like Auction Arms, and Gun Broker. I see them there sometimes. Also Flea Bay.
Thats good to know, don't have to limit myself to a New Englander barrel. The key word in my above post was "cheap", would be nice to get a 50 caliber barrel for it if it was cheap enough.