As a general comment, I suggested 100-grain factory loads partly because the muzzle velocity generally isn't super-zippy out of typical .243 sporters, generally somewhere in the 2800's. This is still enough for a middle-chest hold out to 250 yards when sighted-in two inches high at 100, but doesn't tear up as much delicious pronghorn meat as lighter, faster bullets. And there ain't much meat even on a big buck, usually no more than 40 pounds.
Also, how quickly a certain bullet kills pronghorns is pretty much irrelevant. They live on the wide-open plains, not thick woods. So what if one goes 50 yards before dropping? In fact, we often used monolithic bullets for pronghorns. They may not kill as spectacularly with double-lung shots as plastic-tipped lead-cores, but even if they slip into a shoulder (whether due to wind, the shot angle, or shooter error, which would of course never happen to any Campfire members) the amount of meat-loss is minimal.
