I've loaded 100-grain bullets of various sorts to 3200 fps in the standard Roberts with IMR4350, and had the pressure checked. It was fine, as it should be, since by the basic rules of interior ballistics the .257 Roberts should be capable of more velocity than the 6mm Remington with a 100-grain at the same pressure. There are a bunch of published loads at around 3200 or even higher for the 6mm. Nosler, for instance, lists a top velocity of 3261 fps for 100-grain bullets in the 6mm from a 24" barrel.
Consequently, when I was running a .257 AI (which was a few years ago) I expected a little more velocity than from the standard cartridge. The only accurate Barnes bullet in that rifle (a NULA Model 24) at that time was the 100 XLC, and I used 52.0 grains of H4350 for a little over 3300 fps. Dunno what the pressure was but based on all the usual signs, plus the results safely possible from the 6mm Remington and standard .257 Roberts I had no worries.
