Here's a photo of the cases in question.



http://www.hunt101.com/img/058573.jpg



The case on the left is my 9.53 Lazzeroni HellCat which is the short Lazzeroni Patriot necked up to 375-caliber.



Center case is the 450 Marlin.



Case on right is the 458 WSM which is only slighlty smaller than the Patriot but both have more internal volume than the 450 Marlin.



FYI The 450 Marlin, using almost any 300gr bullet can reach 2500 fps which is the standard velocity and weight for the 375 H&H.l..this assumes a standard throat and a reasonable length barrel....after than you just run out of space no matter how hard you try to compress the powder.



My HellCat will reach 2600 fps with a 300gr bullet so if that case were necked up to 45-caliber the velocity of a 300gr bullet would be higher than can be reached with the 375-cal version (bore-expansion ratio and all that good stuff)....this means that all other things being equal you should be able to get 2650 to 2700 fps with a 300gr bullet out of the 458 WSM and 2350 to 2400 fps with a 350-400gr bullet shouldn't be a problem either. I don't know if I would care to shoot it in a ltwt Browning BLR but that's up to each shooters tolerances.