There was a time when Weatherby brass was soft. What was the date on that reload document? 338 Win Mag equaled or bettered 340 Weatherby. Same with the 300’s.
Look in other (new) manuals you’ll find that the Weatherby loads better the Winchester’s.
It's dated 2013. When I worked up a 180 grain load a few years ago, the Sierra and Hornady manuals both listed 3200 fps maximum for rhe.300 Wby.
The .300 Wby has got to be faster than the .300 Winchester at equal pressures for simple law of physics reasons--case capacity.
Another The manual says 73 grains of RL7 with a 300 grain bullet in the .458 Winchester a compessed load but 74 grains with a 400 grain bullet isn't.
I think the manual is junk. Some of their powders are very good
Old manuals in general are not as good as the newer ones. If you want to make a magnum out of your old war horse look at early Speer manuals (kidding).
However, my old Lyman manuals are about worn out as I refer to them all the time for cast bullet loads.
I’d guess that Alliant had not updated their data since the soft brass of years before.
That soft brass issue caused me to look down on Weatherby at the time and I bought a 300 Win Mag. Now I have two 300 Weatherby’s.
Old manuals are good for some things, such as finding some data on obsolete cartridges and cast bullet loads, but many early manuals were developed without the use of modern testing equipment.
But even using new manuals a reloader has to be careful, first using the same components - brass, bullets, primers. But some powder lots differ a bit. Then there’s differences in chambers and bores.
Besides Gun Gack and Gun Gack II I have several manuals and when starting out with a new for me cartridge I’ll look at several and perhaps old issues of Handloader magazines.
You often see advice on working up loads in your rifle with your components, good advice. I think the advice I’ve seen from a sage in this forum is to measure velocity and not to exceed velocity seen in trusted sources. Velocity, I believe is proportional to pressure as measured by the old crusher method and the measuring of the expansion of the base of the fired cartridge method. That’s about as good as a shooter can do without expensive equipment, in my opinion.
