The .257 Roberts has about a 25% powder capacity advantage over the .250-3000, on average with various kinds of brass, and a 100-grain lead core bullet seated to standard OAL. This means an approximate 8% velocity advantage for the Roberts when the same bullets are shot in the same barrel length, with optimum powders for both rounds. If a .250 load gets 3000 fps in a 24" barrel, the .257 should get around 3185 in a 24" at the same pressure.
rosco1--No, the low SAAMI pressures for the .250 and .257 are NOT due to them chambered in "surplus" rifles. Instead, the .250's is low due to being chambered from the get-go in the 99 Savage, which tended to be hard to open after firing hotter .250 handloads. The .257's is low due to being designed as a woodchuck cartridge by Ned Roberts, who found it more accurate at lower pressures, probably due to the relatively poorly balanced bullets of the day, which did not shoot as well at higher velocities. Remington followed his lead when turning the .257 into a factory round in 1934, and so did SAAMI.
