Yeah, capacity always wins. The .257 Roy has about 80 grains of case capacity, the .25-06 AI about 65.
According to the 1/4 rule (which was stated correctly by somebody on this thread) the .257 Roy will therefore get close to 6% more velocity than the .25-06 Ackley AT THE SAME PRESSURES.
That phrase is capitalized because it is important. Yes, you can get .257 Roy velocities out of the .25-06 AI, but only at much higher pressures. This is generally the trick with making an AI cartridge act like a bigger round: load more powder.
At equal pressures the .257 Roy will beat the .25-06 AI by around 200 fps, depending on the load. Thus if somebody is stoking the .25-06 AI white-hot and getting 3600 out of a 100-grain bullet, then the same "loading technique" would get 3800+ out if a 100 in the .257 Weatherby. And some people do. It's that simple.
It also isn't magic. It's just pushing the cartridge beyond the standard limits. Whether this is smart is debatable.
