Combination guns are compromises, the 243 being a lot of gun for varmints generally and small enough to give spotty performance at times. The 257 is 20% more gun than the 243. I think the 250 Savage and 257 Roberts were developed as medium game rounds first and as varmint rifles as a second job not the other way around.
.257 Roberts was developed as a varmint rifle as there were little to no deer in the east just after the turn of the century. The .30-06 was not "The" deer rifle yet and and game rifles were still fairly large caliber.
