Some more data:
Since the powder capacity of the .257 AI is about halfway between the standard Roberts and the .25-06, we'd expect potential velocity to be about halfway between them. We've already seen that the standard .257 should easily be capable of 3200 or so with 100's from a 24" barrel. .25-06 data from manufacturers who use the same barrel to work up loads and chronograph them indicates 3350-3400 fps is possible. Thus we come up with about 3300 fps in the .257 AI.
Also, the data for Accurate 4350 (which I've found similar to IMR and H4350) shows 52 grains as maximum with the 100 TSX from a 24" barrel, for a velocity of 3293 fps at 61,100 PSI.
