Well I have the 15" Encore barrel and I use 140 grain bullets with IMR-4064 a good medium burning powder.
I mostly don't load anything other than 140 grains, but the 120 grain look promising for Antelope.
Take a look at Hodgdon DOT COM reloading data for 7mm-08 Remington pistol and you will find a loads for the 15" barrel.
I also use once fired 308 Winchester brass civilian or military, because its much cheaper.
Please remember to start at the reloading manual published starting loads and work your way up to find you best accuracy.
I stay about 3 grains off reloading manual published max loads for the thicker military brass.
yooper77