I have a Green Mountian 1-18 twist, but it is a 1" across the flats. GM made this limited run a couple of years ago. I don't think any 15/16 were made. They no longer produce this barrel, and I can't understand why. Mine is an amazing barrel. This gun has a Lee Shavers globe sight with a level. It also has a Lyman 57 SML peep sight and this rifle is sighted in out to 300 yards. This gun shoots a 386 gr Lee 459-405-HB bullet paper patched over 80 gr of Pyrodex P. I also shoot a 475 gr LymanWhitworth bullet that is also a very accurate bullet but the sight is too short to go to 300 yards.
I have seen a couple of these barrels for sale over the last year. I wish I would have bought 2 when I had the chance. Ron