I would think a .243 Win would be a great Pronghorn rifle. I am planning on hunting them this fall with a .257 Wby Mag that I bought and setup mainly for hunting Pronghorns. I have only killed one Pronghorn previously and I shot him with a 7mm Rem Mag. They are relatively small as Big Game animals go and aren't heavily built, so they certainly do not require big bullets or magum power. The only hesitancy I have with the smaller cartridges is their ability to handle the wind that always seems to be howling and gusting in the wide open plains and sagebrush flats where Pronghorns are found.