You should try a heavier bullet. 210 berger, 220 or a 240 SMK. H1000, H870 if you can find it or WC872 which is military surplus. Plenty of it out there and very affordable. WC872 is very close to the burn rate of H870. start low and work up. That RUM will start singing. You'll need a 10 twist for the 240's.