ERK,
Would love to hear what your sources are. Here are the top velocities for 100-grain bullets from the most recent pressure-tested data, with 100-grain bullets:
Source--.250/.257
Barnes--2832/3152
Hodgdon--2936/3205
Hornady--2850/3200
Lyman--3012/2985
Norma--2836/3045
Nosler--2952/3120
Ramshot--2864/3051
Speer--3026/3113
The only source that lists .250 velocities as high as the .257 is Lyman. Their load is 33.0 grains of IMR3031 for 3012 fps--while Hodgdon lists 34.5 grains of 3031 as max, for 2875 fps. Somebody must have been asleep at Lyman's chronograph.
The .257 has more than 15% more powder room than the .250. There is no way the .250 can match .257 velocities at the same pressure. That is not how rifle cartridge physics works.
