ORIGINAL: shepdogwv
I was just wondering if anyone with a reloading book could give me some info. I was just wanting to know what the numbers are for a .257 Weatherby Mag. (117/120 gr bullet) and a .300 Weatherby Mag. (165 gr.) using the same powder and the same amount of powder. My father-in-law and I were just having a dicussion but his reloading books are at his house, and some hard numbers would be helpful. Thanks
The .30 Weatherby is a full-length case that requires quite a bit more powder to get optimum performance vs the .257 Weatherby, which is a "standard length" magnum. If you used the same powder, and loaded eaach with the bullet weights you specify, it will take alot more powder in the .300 WM. So, if you used the SAME AMOUNT of the same powder in the .300 WM, it will be WAY underloaded!
For example, using IMR 4831 (Speer manual), which is the slowest I can find that is used in both:
.257 WM with 120-grain bullet - 65 grains MAX LOAD, MV = 3199 FPS.
.300 WM with 165-grain bullet - 78 grains MAX LOAD, MV = 3142 FPS
IF I reduced the .300 WM load to the 65-grain level, same as the .257 WM here, the MV of the .300 would drop to vic. of +-2600 FPS, or less than the .308 Winchester! NOT a good idea with slow powders in big cavernous cases (ever heard of S.E.E.?? Or D.D.T.?? Deflagration to Detonation Transition.)
One Speer max load uses 89 grains of RE25 in the .300 WM with the 165-grain load, MV = 3258 FPS But this powder is too slow for the .257 WM, I guess, since there are no RE25 loads shown for it.....
Note that the SD for the 120-grain .257" bullet = .260, and for the .308" 165-grain bullet = .248.
Therefore, the 120-grain .257" bullet has more inertia, sois harder to acceleratevs the .308 165-grain bullet.....