I’d rather be “over-gunned” on pronghorn than under-gunned on elk. I’d look at 7mm-08 or .308. Pick a mono metal (Barnes, Hornady GMX, etc). That will help limit meat destruction on pronghorn, while facilitating good penetration on elk. My single rifle for elk/deer/antelope for the past decade is a .300WM and 165 grain Hornady GMX. No issues with meat loss if you stay off the front shoulder on antelope, and plenty of penetration on elk.