I have a custom Sporter built on a Remington 721 action, sporting a Lothar-Walther barrel in a 1-8" twist chambered in 6.5x55 AI. It is bedded into a Grey-Laminate Remington 700 Stock. [Guntech on here did the work]
I am very happy with this chambering. The AI case is a much nicer design for the reloader than is the stock case due to the negligible taper. You do not have to FL resize anywhere nearly as often, and trimming is practically eliminated, unlike the parent 6.5x55.
The gain is about 100-150 fps, depending on the bullet being used. Very, very close to 6.5-284 performance, but with a bit less powder.
My powder choices include: N560, Norma MRP, H4831SC, IMR7828, Reloder 19 & 22.
With the 139/140 grain bullets, 2970 is close to max working pressures in my rifle. It regularly shoots 5 into ½ moa, using a quality bullet.
I believe this is worth doing on a modern action, but am not sure about the surplus actions. Regards, Eagleye.