You probably have the original CIP chamber in your rifle.
I had one like that.
It has the same throat as the SAAMI/CIP .458 WinMag tacked onto the long-brass chamber.
You can load it up and beat the now-standard SAAMI .458 Lott for higher velocity at lower pressure,
just takes more powder to do it.
However, it will give about 2200 fps with ammo supposed to give 2300 fps in the SAAMI .458 Lott.

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.