You might try positioning the powder to the rear of the case to see if that gives more consistant results by moving the lever to within a 1/4" or so of closing, then raising the muzzle to at least 60 degrees momentarily, then closing the lever the rest of the way after the rifle is in the firing position.
I don't have a Rossi in .45 Colt, but I do have a Marlin Cowboy in that chambering. Mine shoots the 250 gr. bullets better than the 200 gr. As part of what Yellowstone Clay suggested, I would suggest that you try some 250 gr. bullets, but in .454" diameter.
My two favorite loads with this bullet are 8 grs. of 231 or 8.5 grs. of Hogdon's Universal Clays. If I do my part, they are good for 1 1/2" 5 shot groups @ 50 yards.
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