I have personally killed multiple pronghorns in Colorado, Montana, New Mexico and Wyoming. The cartridges have included the .22-250 Remington, .243 Win., .257 Roberts, .257 Roberts Ackley Improved, .25-06, .257 Weatherby, 6.5 Creedmoor, .260 Remington, 6.5x55, 6.5-06, .270 Win., 7x57 Mauser, .280 Remington Ackley Improved, 7mm Remington Magnum, 7mm STW, .308 Winchester, .30-06 and .300 WSM. Probably have killed more with the .257 Roberts than any other round—at ranges out to 550 yards, though that was finishing off one somebody else wounded, back when I was guiding.
Eileen has taken them with the .22-250 (with a 40-grain Cutting Edge Raptor at 4300 fps), .243 Winchester, .240 Weatherby, .250 Savage, .257 Roberts, .257 Weatherby Magnum and .270 Winchester. She has also probably killed more with the .257 Roberts than any other round.
Have guided hunters who used the .220 Swift (my rifle, because the guy flinched even with a .25-06, because of too much shooting with a .338 Winchester Magnum), along with the usual suspects from the .243 Winchester to 7mm magnums. My hunting partners other than Eileen have also used various other rounds, including the 6.5-.284 and .280 Remington.
Will also include springbok in Africa, since they are essentially the same animal—just about exactly the same size, coloration, length of “trophy” horns, plus open country habitat and hunting techniques. Have personally used rounds from the .22-250 (out to 500 yards) to .30-06 (on my largest-horned springbok), but due to most safari hunters bring rifles in larger chamberings, have also seen them taken with such rounds as various .300 magnums from the WSM up, the wildcat .338 WSM, and .338 Winchester Magnum.
They have all killed pronghorns (and springbok) quite well.
My vote, based on purely sentimental, and multi-generational reasons, would be the .257 Roberts.
