I've been loading for the .280 for many years, and my favorite load for my old 725 Rem. is 53 gr. IMR 4350, 139 gr. Hornady SP, Rem lg rifle primers. I also used this gun/load comb. with great success on groundhogs out to 300 yds. It also just flat KILLS whitetail.
As with most all guns what shoots good in one may not perform that well in another. You just have to work it out. That's the fun of reloading, and why we do it.