RE: .257 Roberts
The .257 Roberts is a better choice than the .243 because you can use up to 120 grain bullets and still be the equal as a varmint gun!! Mine is a model-70 featherweight and I dearly love it. I have two .25-06 rifles that I use too, but that little featherweight fits me so well.....
If you're into building guns, the .257 is a good choice. One of the real problems with that cartridge is that it's so often chambered in a short action gun. The magazines are real cramped and if you wish to load the 120 grain bullets, they have to be seated so deeply that you lose a lot of valuable powder capacity. The '98 mauser is good for that cartridge.
I see the .260 Remington as the equal of the .257....the .264 bullet being only .007 larger than the .257. And you can shoot much larger bullets in the .260.
The .257 Roberts is a helluva good round.....I'm sorry that it seemed to be "taken out" by the .243/6MM class of cartridges.