I'd still you to take a good look at the bullet. I Now understand that it is a Barnes Monometal. But still look.
We have known for years that some .264 bullets are two diameter. Do the caliper jaw test I mentioned. Is this bullet tapered or two diameter? Those pressure relief grooves are many, and really go up the bullet to its tip. Secondly: how did you determine your seating depth? When I start with a new bullet I take a cleaning rod and run it down the muzzle to the breech face, and run a pencil line around the rod at the muzzle. Next take bullet and drop into the action until if stops in the throat. Hold it up there with a pistol rod, and again place your cleaning rod down the barrel until it is stopped by your test bullet. Draw a ring around the rod with the pencil again. Take your vernier calipers and measure this distance, and consider it your MAX overall cartridge length.
I know all of this sounds over simple, but it seems to me that you might be missing something that just requires another look. This is the kind of thing that is best walked away from and returned to the next day. This would be a good way to get a different perspective.
I'd love to hear what this exercise shows you - if anything. I just find your problem intriguing.
