Back to my 25 caliber inquiry for a CPA barrel.   If what I want to do is launch Sierra's # 1630 117 gr SBT at about 3000 feet why not from a 257 Roberts ? When I say 3000 I mean near 3000 if it chronographs with accurate loads at 2900 FPS not a problem I do want better than 250/3000 balistics

Roberts is formed from 7x57 and 7x57 Rimmed brass is easily available as are Roberts dies.  All it would need is a rimmed shell holder.

On the chamber, I assume CPA would rough out the Roberts chamber. Then open up to accept the rim & finish chamber with a regular 257 Reamer, nothing special needed for the Rimmed version. Extractor is straightforward just like any other rimmed case

It could be throated for that specific bullet overcoming one of the Roberts handicaps, short bolt actions not allowing long nose bullets.   

Any pittfalls you can see ?

Boats

Boats
Why not just jump up to a 257 Roberts Ackley Improved. It will give you the velocity you want.

Boats, there is no problem with your idea. Have who ever chambers it to just cut the rim in (on the face of the barrel) with a boring bar. That's the way that I do all my chambers. The 7mm rimmed has a thinner rim than 30/30 or 30/40 so, make sure the guy has a case available for the head space.

Also, with a 28" barrel, you should be able to attain 3000 or extremely close to it with the std 257. The improve only actually gets about 50 fps more that the std at the same pressure but, burns about 5 gr more powder to do it.

Frank

3000 fps easy with 117 grainers on 25 Ackley Krag in a good strong rifle as the CPA and easier to get brass