I looked through my notes on the .257wby. This is my experience:
I got 3,300fps w/70.0gr of RL25, best load for my rifle with 115gr bullet with Norma/Wby brass. (not partition- ballistic tip.) Don't use the Ballistic Tip for deer at close ranges. Failure to penetrate.... don't ask me how I know..... messy!
My particular lot of IMR7828 is on the "fast" side and requires substantially lower than recommended powder charges, often only 1.0gr above starting recommendations for max. pressure loads from my gun... A Weatherby Vanguard. The only record I have is that I stopped at 67.5gr but got almost 3,400fps, but primers were cratering and brass was imprinting the ejector hole.
I "save" the IMR7828 for my .300RemUltMag. It really "thrives" on that powder with 165 and 180gr bullets....
Most of my loading for the 115gr and heavier with the .257wby has been with WC-860, a surplus .50BMG powder. It is much slower than either IMR7828 and RL25. It is also rather temperature sensitive. However, I have gotten some good velocities and excellent accuracy using reformed Remington 7mmMag brass, Federal 215primers and 76.0-78.0gr of WC-860.
I'd stick with the RL25, it has proven more satisfactory in the .257wby. (besides the aforementioned WC-860). Other powders I've had good experience with are H4831 (duplicates the older factory ammo. Current factory ammo uses Norma MRP, another good powder.). Also, look at Ramshot Magnum. RETUMBO was something of a dissapointment. Not nearly as good in the .257Wby as the RL25.
However, better yet, look into getting some WC-872. It is even slower than the WC-860 I have and should do well with the heavier bullets from the .257wby. Wideners Shooters Supply and GI-Brass.com are sources for this surplus powder.
Before using ANY data recommendations, check and see if your rifle has any "Freebore". My Vangard as do all Weatherby factory rifles have a 0.378" free-bore or throat.
A custom chamber my not have this long of a throat and as such will require substantial reductions in powder charges to avoid excessive chamber pressures
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Also, take note that Federal, Remington, and Hornady cases are considerably heavier/thicker than the Norma/Weatherby brass and will likewise too, require lower powder charges.
Be sure to consult the Speer#14 manual for data. They used a Remington Action/Barrel and Federal cases (besides Speer bullets) to derive their data, and their velocities/powder charges are likewise somewhat lower than most other sources...
You may end up being dissapointed with your decision to rechamber your .25/06. That is unless you have a "Weatherby" free-bore chamber. Without the freebore, you may not even be able to best the .25/06 for velocities from a 24" bbl... You can only burn so-much powder in a .257" bore before you reach the point of diminishing returns.
