The 257 Roberts is a 7x57 necked down to .25 cal and the .244/6mm Rem is a 7x57 necked down to 6mm. That's it, so yes you can neck down .257 brass if you had to. I didn't think there was a shortage of 6mm brass. I know Remington has an 80 grain factory load for the 6mm so that would be a good place to start for the non reloader.
NRA Patron Life Member