The .257 Wby is a magnum cartridge, to be sure, the SAMMI pressure spec is 55,000 cup. Slightly less than the non-magnum .25-06 at 56,000 cup.
The case holds, on average, 84 grains of water. That’s a good heapin’ helpin' in my book. But, in comparison, a .300 Win Mag will hold 89 grs.
Pressure and capacity wise, it seems pretty middle-of-the-road, as magnums go, by today’s standards, at first glance. But…..
What sets this cartridge apart is what’s called Relative Capacity. RC is a function of case capacity divided by bore area. It’s a key concept in cartridge performance predictions and optimal powder selection.
Dude, Relative Capacity for the .257 Wby is off the friggin’ hook!! Right up there with the .50 BMG. Take heed. Slamfire1 ain’t a kiddin’.
So, given what we know about the case, it’s capacity, length, COAL, the OP’s desire to use a 100 or 120 gr bullet…..
Mirror, mirror, on the wall. What’s the best powder of them all?
Well, I don’t have a magic mirror. But I do have my own Powley Computer (Powley-esque, really). Punch it all the numbers, just takes a second (it’s a spreadsheet, so it’s just a quick cut & paste, enter the bullet weight & length, COAL looks good, a little tweak to the load density…..).
Yep. Big surprise here. You gonna need a really, really, REALLY, slow powder. I’m gonna recommend you start with RL-22 for the 100 gr bullets, and H870 for anything heavier.