My main hunting rifle is a 257 Roberts in a Model 70 Featherweight. I load somewhere in the middle of what most reloading manuals say.
Never had a problem with accuracy and never had to shoot a critter twice.
I just don't see the need for hiper velocity in a 257. Also I don't see any reason for deer hunting past 300. I would extend that a bit for antelope but most are under.
I go for accuracy not velocity. At 300 yards you wont see the difference of 150 or 200 fps MV on a 25 cal bullet.
Sounds like you're trying to make a 257 Magnum.
If you need to push that bullet farther, look at the 25-06 or something.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071