...for the .260 rem, it would just require re-barreling, the 6.5x284 will fit in a short action, but just barely & requires seating the bullets way back to fit the magazine, or lengthening the mag to use the longer, higher BC bullets. Another option is the 6.5x47, a reworked .260 case w/ a small primer pocket, less case taper, set back shoulder, & longer case neck. Pretty much cures the problem w/ COAL you might run into w/ the .260 & seating longer bullets, it was developed as a international competition round w/ very impressive ballistics...