You are right about the Barnes X - they group very well in some guns, but very bad in others. I can't explain why, but I know they do.
In my 300 Win Mag, I can place 3 Barnes X so tight that they touch each other. The other thing that you should know about the X is that they generally need more velocity to open up. As such, you should use lighter bullets. However, this should not be a problem because the X bullet maintains very, very close to 100% of its weight. In my .300 Win Mag, shooting a 180 X is like shooting a 200 Nosler Partition, or even a Trophy Bonded. Even in my .30-06 with 165 grain Barnes X, it has ALWAYS fully penetrated deer and hogs. On just an educated guess, I would think that a .264WM (of which I also own, but have not yet shot) should shoot Barnes X in the 120 grain range or so.
Oh, BTW, remember that my .300 also has the BOSS. I forgot to say that, at least in my gun, the 180 grain Barnes X was VERY sensitive to BOSS adjustments. Keep it set at its desired setting, and its accuracy is .5 MOA.....move it just a bit, and the groups open up to over 1.0 MOA.
Zachary