Well, it is pretty much a given that the Weatherby loads are surely loaded with one of the excellant Norma powders.
Which one would be the problem to figure out. Besides, they may be loaded with a powder formulation that is not even available to the reloading public.

Factory Weatherby loadings in the .257 Wby Mag have always been excellant in regard to accuracy and velocity.

I have seen several guys who could achieve the accuracy of the factory stuff, but they were always a little short of the factory velocity with safe loads. Not that their loads were not plenty fast, they just could not get to the level of the factory stuff.

It is entirely possible that your rifle just prefers the Hornady bullet style better than the Sierra's. You should try reloading some Hornady bullets with something on the order of RL 22 or RL 25, etc.

My dad shoots a .257 Wby Mag and has great success using the 100 grain Nosler Partition in his reloads. That bullet would likely hold together a little better at the extreme speed possible with this cartridge.

You will just have to experiment a little or pay the price for factory stuff.

R F

Here is some information from the old Weatherby Guide 20th edition. Use at your own risk.

look at the pressure with standard powder and then the morma for the 87 grn bullet. how do they do that?

.9 grains less w/only a 50fps loss and they drop pressure by 7,000!

love the 100 sp in my 257 roy. way beyond what i would expect out of handloads much less factory. to bad they cost so freakin' much!!!

woofer

Thanks M16! That at least gives me someplace to start.

Good point Woofer! 7k of pressure difference is huge. The factory loads didn't shoot as well today as they did yesterday, but still better than my handloads. I'm gonna stuff that case full of RL-25 this week and see what happens. Maybe I can keep the pressures down and still get the velocity. One thing I have noticed. When I was seating the bullet out as far as possible (there's tons of freebore in this rifle) I was getting powder blowback around the outside of the case all the way down to the belt. Went back to SAAMI specs today and no more blowback. I guess the case wasn't expanding quick enough (low pressures?) to the chamber walls with the bullet only seated enough for the case to hold it. I've never seen that before.

You can reproduce the speed and accuracy with Reloder 22. I got 3630 fps (100 gr Hornady) and 2" groups at 300 yds all day long.

The 257 Wby Factory was loaded with MRP.Whether it would be "canister" grade MRP,who knows. RL 25 and 7828 both shoot very well in my 257 Wby.Rl 25 gets the nod in accuracy and velocity but not by much.

I'll second the Reloader 22 recommendation. I've had good luck with 72.5 gr R22 with Barnes TSX 100 gr. I've also had good luck with R22 and swift a-frames.