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

Loads for the .458 B&M are good in the .458 WM with 0.260" longer COL for equivalence.

Surprising thing for me in the table above:
Different bullet types with same weight differ wildly in same rifle with same powder charge for delta Velocity per inch of Barrel Length,
dV/dBL.
Not to mention that all three rifle barrel lengths (18″, 20″, 24″) are from different rifle barrels,
and all that entails.
Complex !
Whatever gets the largest area under the time-pressure curve in the least time is going to lose less velocity per inch in a given rifle ?
Just a SWAG.
Factors like bullet material and bearing surface area,
barrel bore and groove diameters and bore area and rifling twist and finish on the rifling surfaces, etc.,
chamber slop and throat variations, etc.,
assuming identical loads fired in all the rifles, still a lot of room for variation in the dV/dBL.

My Miroku M1885 .45-70 Gov’t. has 0.457″ groove diameter, 6-grooves, 1:18″ twist, for example.

I guess the MIB rifle was a re-barrel of the M1885, for .458 B&M with 24″-barrel length ?

Best to start with one barrel and compare it at various length cut from long to short, nothing else changing,
including throat.

Confession:
My .458 B&M+ started off with a request for 0.400″ length of PSFB, and it had a 19.75″ barrel.
The extra Parallel-Sided-Free-Bore of 0.400″ would be expected to lower velocity (and pressure) for the same load
as tested in the MIB 20″-barreled standard chamber (short throat), and it did,
more than the trivial 0.25″ shortening would cause.
My 19.75″ velocities (.458 B&M+ 0.400″ PSFB) with AA-2520 77.0 grains, at 60*F:

450-gr CEB BBW#13 FN: 2203 fps at 5 yards, for BC = 0.190 corrects to 2224 fps MV
420-gr CEB BBW#13 HP: 2223 fps at 5 yards, for BC = 0.180 corrects to 2246 fps MV

MIB instrumental velocities of 2249 fps and 2254 fps respectively, if 5-yards chrono,
would correct to MVs of 2271 fps and 2277 fps respectively, at assumed 70*F standard laboratory conditions.

I got more difference between heavier FN and lighter HP.
Throat effect ?
My twist of 1:10″ instead of 1:14″ is another trivial factor, less important than throat.

The 16.875″ barrel has been throated a bit longer still, with a .458 WinMag throat.
That is the confession.
New baseline for my rifle. Will make allowances for that.

Just have to shoot it to find out the difference with barrel shortening.
This prediction stuff will give one a headache.