UPDATE...

I just finished shooting this afternoon in my little range behind the house. I tried the .29 cent rubber O-ring trick and tweaked the powder charge on the load I've been working with a bit too in the process...

550gr Woodleigh Weldcore SN @ 1825fps (I'm not looking for maximum, just accurate - this is plenty for ANY North American game!) turned in a 50 yard group off the bench...

Center to Center measured with my dial caliper = .45"

To say I'm pleased would be an understatement!

Thanks guys, I've got more tinkering I want to do, but this is phenomenal to me for a heavy caliber rifle. Though Elmer Keith said many times that his two #1's (in .375 H&H and in .458 Win) were the most accurate rifles he owned -both turning in "one ragged hole at 100 yards" with factory 500gr ammo.