hey Byron,
Seneca barrels are 13/16ths across the flats. all the barrels I'm aware of that will drop-in yer gun are 32cal, 36cal, & 45cal. a 50 would hafta either be a stock barrel bored out or made from a blank if ya can find a spare breechplug(if the threads on the breechplug are big enough to thread into a 50 blank). luck & have a good'en friend, bubba.