I own a .280, my 2nd, and have owned two 7 mm Magnums as well. I have not noticed any correlation between the two as to accuracy. Unless the rifle has something wrong with it, lousy bedding, rough barrel crown, a sloppy chamber, either should do that well, assuming a reasonable quality bolt action or a single shot. Auto loaders and pumps, no. I had a BAR in 7 mm Mag that shot 1.25 - 1.5 MOA, however.
Some rifles are finicky as to what they like and how many loads they shoot well. The lighter the barrel, the fussier they tend to be. E
