MHB - about your rifle's bores.
: Is seems common, I guess, that the European's make groove Dia. oversize with deeper than nnormal rifling. My .6.5 is that way, yet it shoots Barnes"X" bullets well, even though they shouldn't shoot in the loose groove dia. When close to standard (I thought that was .510", not .512") the bore's dia. is very important as the deeper the rifling, the more bullet material is moved. O'course, you probably already know that. When shooting such large charges, any blow-by from loose fit might be very bad for the chamber throats, especially with 4831.(very hot burning)
: I wonder if you could start and use a 1/10" card wad between powder and bullet. Would that not help if there is blow-by? Perhaps this isn't necessary as you might not be shooting many thousnads of rounds. Any wad must be HARD up against the bullet, not down on the powder if not a compressed load.