Most load programs say that 50,000psi is the limit. Is that the case with Weatherby rounds?
Absolutely not. Unfortunately SAAMI does not list data for Weatherby cartridges but most reloading manuals list MAP somewhere between 60,000 and 65,000 PSI.
Weatherby factory ammo is said to be loaded to 71,000 psi.
Extremely slow burning powders are going to give you the most velocity in the 270WBY.
Reloader 25, Retumbo, US869, Ramshot magnum, H1000, H4350,
IMR 7828.