Looking at a rifle offered up on another forum, I have a 6.5 x 280 AI and can't see anything gained other than the shorter action. Maybe I am missing something?

The 6.5 PRC has better case geometry with slightly more case capacity. The PRC case is more efficient with slightly more case volume.

^^^^ agreed + factory ammo in PRC and of course your short action you already mentioned.

And if you want to wring out the full Potential the 6.5 prc should be built on a long action. Like the saum or wsm case. It’s amazing what the 300 wsm or 7 wsm is capable in a long action where you don’t have to stick the heavy and high bc Bullets over 1/2 way down in the case. The PRC is marginally faster than any 6.5x284 or 6.5-06 std or AI I’ve owned.