The 400-gr #13 FN from CEB has been test-fired in the .458 WM and .458 B&M.
Note the COL used for the .458 WM was only 3.185".
The COL used for the .458 B&M was not revealed.
If 3.185" is possible with 1 band out of case, as Doc M specified with the .458 WM,
then with 2 bands out of the case a COL of 3.285" should be the ideal loading length for the .458 WM.

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Comparing the .458 WM to the .458 B&M, both with 20" barrel, using 400-gr CEB #13 FN:

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Ron aka "Rip" for Riflecrank Internationale Permanente
NRA Life Benefactor and Beneficiary
.458 Winchester Magnum, Magnanimous in Victory
THE WALKING DEAD does so remind me of Democrap voters. Donkeypox.