Well I finally got out to the range this weekend with my 41 magnum Ruger BH 4 5/8" and some 240 grain bullets cast with my Lee 90332 mold and loaded over a mid-range load of Unique (should be just about 1,000 fps) they shot a 6 shot group at 2" at 25 yards. Not a tack driver but as I shoot it more I'm sure the group will shrink a little. Should be "minute of deer" out to 50 yards or so. I cast the bullets using WW and 2% tin. Charlie
It may well be worth giving the bore a very complete cleaning with a good copper cleaner to be sure all traces of copper jacket are gone. Some guns are very picky & will not shoot their best until all copper is gone.
Years ago my shooting buddy had a BH in 41 Mag & had this problem, until it was completely free of copper fouling.
He got a Lee 240 gr mould, and I presume it is the same as yours. At any rate he worked up several good loads between 8.0 ~ 10.0 gr of Unique. I would also consider using a harder alloy, if you see any leading. Another thing to try is sizing to a different diameter. Tradition has it that .001 over bore size is the best place to start, but it is not always the final word.
I expect you will be well pleased, once you find the magic load.