I am beginning a new project barrel for my encore. This will be an 18 inch standard taper with a matt blue finish. The primary purpose will be whitetail deer in close cover and therefore will be wearing a trijicon dot sight.I had originally planned to use the 260 remington, but am wondering if I can get better case life out of an ackley improved version. Am I right in assuming that the 40 degree shoulder will retard case length growth? The brass I will use is lake city match and I will be turning case necks after the initial sizing operation.
Rick

The problem with the Encore is that you will not have a rimless improved cartridge but a wildcat with headspace as the barrel cannot be set back as it is with threaded barrels.

Cases can be made to fit from a larger case as you mentioned.

The way to handle case life is to load on the light side. The benefits of a sharp shoulder and minimum body taper are exagerated.

In summary: Leave your 260 chamber as is and load it on the light side for better case life.