I've owned several 44 Mag lever guns with 1:38 twist, and they never met my expectations. I've researched this a bit, and IMO the poor performance (with cast bullets and cowboy loads) is in large part due to the rather lazy 1:38 twist. This seems to be an affliction mostly to those rifles that follow SAAMI specs. IMO, the SAAMI spec for 44 Mag Rifle was written by Marlin, calling for 12 narrow and shallow grooves. The "Ballard" version uses 6 slightly wider but equally shallow grooves.
I had a beautiful anniversary B92 Browning, it was a shallow slow twist rifle, and I traded it off. My next nemesis is a marlin 1894, 6 shallow grooves, and not a tight bore/groove either.
For S&G I ran some of my 44 bullets in an online twist calculator, No Sir, 1:38 twist sure doesn't seem to be the way to go.
I've owned two Brazilian 44's, one a 44 mag and the other a 44-40. The twist on each was app 1:20, at least visually wasn't close to 1:38. The guns had issues, after all they entry level firearms, but bullet stabilization and accuracy was not an issue. I checked the specs on the Italian rifles, no 1:38, or anything approaching that. Not for 44 Mag, 44-40 or 45 Colt.
So I checked the CIP specs, no sir, no slow twist there.
I was so frustrated with shooting cowboy loads in my Marlin, I tried some of my 429360 bullets, and they were the best loads of the week. Really, they wouldn't do squat in my Ruger Vaqueros, really messed with my head.
As soon as I sell my Marlin, I'll be buying an 1873 in 357/38, AFAIK, SAAMI hasn't messed up that caliber yet.
I'd like to know if anyone with a 44/1:38 thinks this is a good thing, because I'm not seeing it.
PS: I must have looked like a lunatic at the range, testing hundreds of combos, in my 1894 Marlin with a Bushnell Elite 3200 10x40mm Tactical scope mounted on it.
My best combo was with a swagged Speer 240 gr SWC and Trail Boss. 2" at 50 yards, and 2" at 100 yards. I suspect that the parallax on the 3200 10x is not that good at 50. It was rather comical, cranking in a wack of elevation to go from 50 to 100.
BTW, I find the 3200 10X pretty handy for testing loads in hunting rifles, a pretty rock solid scope, easy to work with, and when you crank in 3 moa, it moves 3 moa. It also fits on most rifles without too many issues.