Didn't think of it before, but have you tried different powder charges with that load? My .50 has a 44" Colerain barrel; and likes 65 grains of 3f. 5 grains either way results in groups being almost two inches larger at 50 yards (from the bench). I previously owned another .50 with a 38" Colerain barrel, that liked 60 grains of 3f. With both guns I shot/shoot a .018 ticking patch, with a .490 cast ball, and Track of the Wolf's Mink Oil to lube.