I get similar results with my Marlin .44 MAG--The 1-38 twist works well with 180 and 200 gr HNDY bullets--I am currantly using full load of W296.At 50 and 100 yds.
My luck with Speer 240 semi jacket HP and 240gr HNDY-SIL has been good -but not as good as with the lighter bullets. I have been using a 200 gr hard cast bullet with drylube and 2400 and have had good l luck at 50 yds, but have not tried 100 yds.