I have loaded for several rifles of each chambering, and accuracy seems to depend more on other factors than the cartridge--just as it does in most cartridges of the same approximate power and bore size. If you consistently find the .280 tougher to get to shoot than the 7mm Mag I'd begin to suspect something's up with the .280 dies.

“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck