257STW worth a look too depending on what you can find for brass. Basically the same thing minus the double-radius shoulder.
Oh I've considered it. But I'm probably gonna go .25-300 wby for a few reasons - or just leave the existing barrel alone as the 25-06 AI it currently is. It's a very lightly used (throat in great shape) ultra heavy 32 inch bartlein stainless barrel I got on an auction. Just a barrel with a tentative plan for now, things gonna be tight for a bit but within the next few years planning to go ahead and do a cheap build off a savage long action as a single shot, totally impractical and just a pure exercise in hot-roddery.
I haven't fully ruled out either of my other contenders but if I do rechamber this barrel it will be to either a .25-300 wby, .257 STW, or a .25-300 RUM/.257 Allen magnum type cartridge. I've ruled that it will be a full length magnum to clean up the existing 25-06 ai chamber without any setback required, and with the standard .532 (I think) magnum bolt face. So one of these three I mentioned will fit the bill.
It is a 10 twist and as such the 130+ grainers are off the table. If they were not I'd probably go with the full RUM case. But I have my doubts about it beating the wby/stw case by much if at all with 115 grain bullets and lighter just seeing how Fast we can go. I could be wrong tho.
There are a few reasons why the .25-300 wby really is the direction I'm leaning HARD towards over the .257 stw.
1. Brass. This is the big one. Peterson makes 6.5-300 wby and .300 wby brass. Neither Peterson, lapua, or adg make stw brass. Norma makes brass for both, it's not as tough but very volumous and consistent, good stuff just don't get the case life at high pressure. But I will say even there Norma 300 wby brass is waaaaay more findable than the 7stw stuff. And I know you could size and fire form brass from .300 weatherby or .300 Hh or others to make stw brass…this barrel will be short lived as it is, if it requires fire forming it's not happening.
2. Dies.
But anyway, the main idea is that it would be sooooo easy to just run 6.5-300 PETERSON brass to neck and shoulder size in my .257 weatherby dies which I already have. Could also run it through my 450 ackley dies with the expander/decapper removed to resize the full length of the body, no new dies needed.
3. I like the weatherby shoulder and believe it does have some benefits. I don't know that I believe it somehow increases velocity as Roy claimed, I think it's been pretty well proven it doesn't matter either way (much the same as how the few extra grains in an ackley improved case don't magically make it 300 fps faster AT EQUAL PRESSURE) but what I do think it does is sort of smooth out the pressure curve and most importantly I see a strong case to be made that this shoulder geometry should actually help to alleviate the powder bridging issues that often plague extremely overbore sharp shouldered smallbore magnums. And it looks killer, the full length weatherby magnum case is curvy with long legs haha. I'd plan to give this the generous weatherby freebore treatment too. It'd be a pretty different animal than the 6.5-300 despite only being .007" smaller bore because of the 10 twist and greater freebore I think I'll run…this will be a dedicate light bullet going stupid fast just for fun gun. The ultimate point blank ranger.
Now the weak point is the reamer…dedicated .25-300 wby reamers aren't something in many gunsmith's inventories or available as rentals. They are pricey and have wait times. HOWEVER.,..I've never done this before but am actually wondering if it'd be as simple as using the kind of floating chamber reamer that has interchangeable pilots and just "interchanging" the pilot on a readily available 6.5-300 wby reamer with a .257" pilot?