For 100 grain bullets a 1-12 and even a 1-14 will usually work quite well, other than ELD and VLD types. The 1-14 twist can be hit or miss with SPBT bullets of that weight. Flat base will stabilize very well, as long as your velocities are over 2700fps.
I've built several rifles chambered for the 257Rob, as well as having factory chambered rifles.
The rifle with the shortest barrel was a lovely little M70 FWT. It only had a 20 inch, #2 contour barrel, with a 1-12 twist rate but it shot extremely well with all shapes of 100 grain bullets.
The load I used was 42.0 grains of W748, over CCI 250 primers, under 100grain Hornady #2540 Interlock, for 2800+fps. This was a Hot load but OK in my rifle.
The longest barrel I used was on a Ruger No1, 26 inches, with a #3 contour and a 1-10 twist rate.
The bullets I used for this rifle were the same as those used in the M70. However, I changed the powder to 46.0 grains of W760, over CCI 250 primers. This load shot consistently into 1/2 moa groups out past 300 yards, at 3000+fps at the muzzle.
I wanted to get a bit more out of it, for whatever reason seemed relevant at the time, so recut the chamber to the 257AI. I gained another 25fps with the same powder and even lost velocities with some of the others I tried. It was still very accurate though.
My present 257 Roberts is a Remington 700 Classic, 22 in bbl, 1-10 twist rate and it will shoot all bullet weights and lengths well, right up to the 120 grain weight.
Velocities with 100 grain bullets are right around 2950fs with the same load of W760 as above.