The two most popular "Improved" .257 Roberts cartridges are the Ackley and RCBS versions. They are quite similar except for the shoulder angle, which is 40 degrees for the Ackley version and 28 degrees for the RCBS version.

The Hornady Handbook and Nosler Reloading Guide show maximum reloads for the .257 Improved (either version) achieving muzzle velocities (MV) of about 3600 fps with a 75 grain bullet, 3400-3500 fps with an 85-87 grain bullet, 3200 fps with a 100 grain bullet, 2900-3000 fps with a 115-117 grain bullet, and 2900 fps with a 120 grain bullet.

The 115-117 grain bullets seem about right for this case capacity to me. The fifth edition of the Noser Reloading Guide shows that 43.5 grains of RL19 powder can drive a 115 grain Nosler Partition bullet (BC .389, SD .249) in an Ackley Improved case to a MV of 2672 fps; 47.5 grains of RL19 will take the same bullet to a MV of 2960 fps.

Note: A complete article about the .257 Roberts Improved can be found on the Rifle Cartridge Page.