If you've never used Cerrosafe, just remember that there is a somewhat narrow window of time in which to measure your chamber casting. If you wait too long, your measurements won't be accurate because the Cerrosafe casting continues to shrink as it cools.
Another alternative, since you say a 6m/m Rem. seems to be a very close fit, would be to load a 6 m/m Rem. case with a fire-forming load. Since you don't know exactly what you have, I'd stick with one that used a relatively fast burning powder topped with dacron or Cream of Wheat filler, rather than firing a round loaded with a .243 diameter bullet. Of course, you'd still have to slug the bore to get a true land and groove measurement. The fire-formed case could also be used to send to RCBS etc. to have reloading dies made if it turned out to be a wildcat. It does sound like you have a .257 Roberts, but chamber reamer dimensions did vary somewhat before the round was standardized.
Better safe than sorry. Years ago, I was totally blind for about 30 minutes after I had a complete head separation with a sporterized VZ-24 in .22-250. It felt like getting punched in both eyes by Mike Tyson. The problem was old brittle brass from a friend's gun with a minimum chamber, fired in my near maximum length chambered gun. No problems with the same load and new brass. Thank God for Paul Mauser's redesigned bolt shroud in the 98 which deflected gasses above, below, and between the shooter's eyes. I had the powder burns and brass particles to prove it. Wear those shooting glasses or better yet, do any experimentation with the gun tied down to an old tire and a long string to pull the trigger. A large oak tree between you and the gun may be prudent.
