A heavier bullet will stabilize and be more accurate.
I worked at SW part time for a few years and many hunters would come in cussing their ML rifle. They had the same issues, sporadic groups and best grouping seemed to be 10" at 100yds. They were all shooting a light projectile with magnum powder charges. I showed them the .44 300gr. XTP using the green sabot and a load of no more than 110 loose powder, or, 100gr. pellets. They came back and said the "recipe" I showed them took care of the grouping problem.
Like Critter mentioned....If you use the lighter bullet, drop to 90gr. volume powder charge. Cant hurt seeing you have the bullet and don't have to search for the 300gr. bullet.
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