Or he prefers a cheaper, alternative method. There is no need for copper or partitioned bullets. There are options.
In short, he can use what he wants.
Or he has no idea how well cup-and-cores can work in smaller-caliber cartridges. Instead he assumed they won't work as well.
Dunno how many deer and antelope I've killed with cup-and-cores not only with the .257 Roberts but the .220 Swift, .243 Winchester .250 Savage and .25-06. Could look it up in my hunting notes, but am sure its close to 100.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.” John Steinbeck
