You're not being fair to Craig. He definitely used to be a magnum guy for elk and African plains game, but finally decided he needed to actually kill an elk with a .270 Winchester if he was going to keep talking about it. So he did, taking a .270 with handloaded 150-grain Nosler Partitions on a Whittington Center hunt for a big bull a while back. He made the longest shot he'd then taken on a mature bull, a little over 400 yards, and the bull also went down quicker than any bull he'd shot before.
After that he watched his daughter shoot a bunch of plains game successfully with a 7mm-08 Remington, and as a result of both experiences has modified his stance on magnums.
From what I have heard over the years, hunters tend to classify game as "tough" if it tends to travel a long way after imperfect hits. I would put both elk and pronghorn in that category.
Once in a great while any animal will decide not to die in the conventionally quick manner from a good hit. I've even seen that in animals other than pronghorn and elk, including mule deer--and once saw a springbok, an African gazelle about 3/4 the size of a pronghorn, go well over 100 yards with a chest-hole from a .375 H&H bullet you could stuff a football inside. But those are anomalies we simply can't explain.
But I will say that if pronghorns were the size of elk there'd be people advocating .375's for hunting 'em.
