Most of my varminting was done with a built up Mauser. The barrel was a 1:12 Douglas #5 weight barrel. Actual chronographed performance had a 75 gr. Hornady at 3500 fps. The expansion was truly explosive. The trajectory of that bullet at that speed compared favorably with high end performance of the 220 Swift with 55gr. bullet.
Ned Roberts had in mind the 257 Roberts as being a dual purpose round. That is, for varmints and larger Easter game. The Roberts does very well as a varmint cartridge. I'm like most Roberts partisans. I don't want to hear any crap about new cartridges from the middle 1950's. None.