That's strictly a round ball barrel. In a .58 there's no need for anything else anyway. The 29" long .58 GM "replacement barrel" on one of my TC Hawken stocks has the same 1:70 twist. Took a nice doe with it last season as reported here:
http://www.huntingnet.com/forum/blac...wken-wins.html
That 42" barrel would make a heck of a flintlock. Are you thinking about a build
Pluckit?