Mackie. Can't say anything about the X bullets. The 300 gr Sierra bullet is another matter. very accurate in a .375x338 Mag. wildcat and also accurate in a Ruger #1 in .375 H&H. BTW. The wildcat duplicates the H&H in a 30-06 length action, and seems to kick a bit less.
it's one of my favorite rifles.
Paul B.