The short answer is yes. I sold my 260 and have necked down my brass to shoot in my 243. To be sure you're OK with the necked-down brass, measure the neck O.D. of a 243 case fired in your rifle and compare it to the neck O.D. of a piece of 260 brass necked down and loaded with a bullet. If you have enough clearance, you are good to go. You might even see an improvement in accuracy if your chamber's neck area is a bit sloppy.
