Your right, Billinghurst actions are extremely simple, so simple in fact that anybody with moderate experience with hand tools and some casehardeing compound can easily build one from scratch. The book Gunsmithing Tips and Projects has and article by Henry Beverage on building such and action. If you are not up to the task The Log Cabin (a muzzleloading supply house) had some kits some years ago for this underhammer. I ordered one a few years ago and built the action, it was OK, but I could do better building it from scratch, maybe if time permits I will do that. The Log Cabin may still have some kits available, but they were in short supply the last time I looked at their catalog. You still have to temper the spring, but that isn't too bad if you use the oil burn method, otherwise a good heat treatment oven will ensure satisfactory results. As far as the barrel goes, Track of the Wolf (www.trackofthewolf.com) offers precision barrels with false muzzles for reasonable prices that would work well for an offhand underhammer target gun. These barrels are tapered (just right for offhand) and I believe are .38 caliber. Really big bull barrels like the ones used by snipers during the Civil War and featured in that great short story about J.M. Pyne "The Madman of Gaylords Corner, have to be custom made, though I've seen ones 3" across the flats with the false muzzle included for as little as $400.
