Lapua cases, CCI BR-4 or Federal 205M match primers, Berger bullets/follow their recommendation(see website) on twist. Vhitavuori N-140 or Varget powder. Redding full length bushing die and benchrest seater die. If it don't shoot it ain't the load!
I have been using VV N110 lately in my large bore magnums. A bunch behind a 250 gr. WFN GC from Leadheads clocks 1437 fps out of my 7 1/2" Redhawk with amazing accuracy. Who said a .41 kicks less than a .44?
First insure that your extractor is not loose. Grab the knurled section and try to rotate it. Secondly, check your forcing cone at the six o'clock position for cracks. I hope for you that you don't find one, I did!