Came into some Magnum Large Rifle primers and was wondering, assuming you rework the load correctly, is there a disadvantage to lighting your .260 loads with a Magnum primer?

I have never used a magnum primer with a silhouette load, however I read from one of the gun scribes at one time, that if you are burning over 50 grains of powder that you should use a magnum primer, or if the weather is real cold to use a magnum primer. I used over 50 grains in a Rem .280 and a Win .270 and have used both a standard primer and a magnum primer and couldn't tell the difference. The article did say that the mag primer would be worth about 1 grain of powder difference and if you were using a max load, to back off as such. I wasn't using a max load either.

Put a target out at 300-400 yards and shoot some groups while running the rounds over a chrono.

If your SD/ES is low and the accuracy is there, use that primer.