Home Forums Hunting & Shooting Express Rifles and Big Bores Only The great 458 Winchester. Everyone should own one at least once

Sirs,

Nice link on a case capacity reference from Sir Khulu,
but the author notes they are approximate, and "The values are averages from several sources."
Still it is excellent reading and instruction on technique for measuring case capacities oneself,
and it's the way I do it.

Here is a very specific case of case capacity comparison for the .458 B&M to the .458 WM.
My technique was same for both, done at same sitting.
One lot of brass for each, and the final result was corrected from the average length, shorter, case length actually used
(once-fired brass with spent primer sealing bottom of case)
to a specified maximum case length desired for comparison.
This uses a .458"-diameter right cylinder of water 1.000" tall with weight of 41.66 grains water to fill that volume.
Therefore, each extra length of case neck of 0.010" adds 0.4166 grains to case capacity.
True that, in this case of cases.

As originally conceived, the .458 B&M was to have a maximum case length of 2.295" and COL max of 2.995", IIRC.
In practice it had to be shortened to 2.240" max brass length and it still works within the constraints of 3.000" magazine box length.
It also had a very short throat.
Almost no throat.

Average for 10 cases of .458 B&M from Quality Cartridge:
2.295" brass = 97.3 grains water
2.240" brass = 95.0 grains water
The .458 B&M brass formed from shortened .300 RUM cases from R-P have same case capacity.
Same flavor of brass.

Average for 10 cases of .458 WM brass from R-P:
2.500" brass = 95.3 grains of water
2.490" brass = 94.9 grains of water

So for all practical purposes, case capacities are identical for
.458 B&M at 2.240" brass length
and
.458 WM at 2.500" brass length

Just call it 95 grains water, like the default value in QuickLOAD for 2.500" .458 WM brass.

Norma .458 WM brass actually measured was 99.3 grains water.
Norma .300 RUM brass turned into .458 B&M might also be bigger.

Peterson Wildcat Tube brass once-fired and corrected to 2.500" length held 93.8 grains water.

That is a good relative comparison, as good as any, not absolute, just relative.
That is no theory of relativity, it is Viking Law.
So, when loading the same bullets in those two cases of those trims,
both will have the same net case capacity when the .458 WM COL is 0.260" longer.
A .458 B&M with COL of 2.995" and a .458 WM with 3.255" have same net case capacity.
A .458 WM with 3.340" COL will have a greater net case capacity by 3.541 grains of water.

To equal the case capacity of a 3.340" COL .458 WM with same bullet, the .458 B&M+ (long throat) must be loaded to COL of 3.080"
The .458 B&M+ loaded to 3.340" COL has same case capacity as the .458 WM at 3.600" COL with same bullet, same seating depth.
Both will beat a SAAMI .458 Lott at COL 3.600" with same bullet.
Higher velocity or lower pressure or both.