If you had enough dies, you could form the Roberts from the 8x57 brass which is the parent case. I would plan on neck turning.
I've tried that... it kinda didn't work out so well...necking up the Roberts or 6mm Rem is a lot easier and works out better...
8 x 57 has that short neck which was a real forming PITA....
I formed some 257 Roberts brass for an older friend who is no longer with us...
he wanted some Roberts brass that was nickle and couldn't find any...
but he had about a 100 nickle cases in 25/06 from Remington..
so just using some of my 257 Roberts dies, lubed them up.. sized them down, trimmed the neck..
and presto...257 Roberts Nickle brass....
did the same with Winchester 280 brass which is all nickle.. but this time, in a 8 x 58 die, to bump the shoulder into the right-ish place..trimmed and then ran into a 7 x 57 die and presto.. nickle 7 x 57 brass...
some people hate to do that kinda work.. me tho, with the wife engulfed in the TV nightly, I just go out to my reload bench and keep myself busy, while listening to talk radio, or old time radio broadcasts.. keeps me out of doing some of my other more expensive hobbies...like building model airplanes...and then generate too many with no place to put them...