Anyone ever make 257 roberts brass outta 6mm brass?

Yes I have, and I split a lot of necks. I think the brass was old and once fired. I loaded a few of the ones that survived and they performed the same as regular brass. If I recall they were Remington brass.
I use a carbide neck sizer ball on dies to make .308 cases into .338 Federal. Have not had a case necks split with Lapua brass.
Yep I've made 257 Rob brass out of both 6mm Rem & 7x57.

When necking up cases of unknown origin I anneal first.
I do the same to make 6-284 brass. Neck down 6.5-284. Also made 338-06AI necking up 30-06. If necking up its best to have a tapered expander. And lube the insides of the neck as well. I use Imperial neck lube.
Should be fine. The parent case for both is the 7x57.

I don't understand because 6mm Rem brass is much harder to find than .257 Roberts? Plus they are only .010 different in the neck. I go up .030 on the .338 Federal without any issues. Dimensionally tour two cases are nearly identical.
I made 257 brass out of 6mm brass and had trouble, split necks and early case head separation. After reading some it looks like people had better luck necking up to 7x57 and then making a false shoulder with their 257 dies to prevent stretching.