Big Bore Semi's are loads of fun. I have a couple of different 458s, and one .500 that we built. The two big ones are called 50 B&M SA and 458 B&M SA...... Those are 2 inch RUM cases. Both of these are built on AR 10 rifles, and have 18 inch barrels. They are beasts, at 10 lbs.... Way to big and heavy to tote very far. They do shoot great however and deliver big power down range with proper bullets.....
The 50 can run the 300 Hornady to 2400 fps, 335 Raptor to 2300 fps, 375 Solids to 2140 fps, and 400s to 2050 fps......
I really like the 458 version, but we have a very serious bullet from Cutting Edge, the 250 Socom Raptor... originally designed for your Socom at 2100 fps. In the 458 SA we run the 250 to 2675 fps with 64/AA 1680, 300 Gr North Forks to 2400 fps, 300 Barnes TSX to 2400, 325 Solids to 2325 fps, the Hornady 325 Flex to 2350 fps, and various 350s to 2250 fps. I have used that 250 Socom Raptor in Africa in 458 B&M Bolt guns at 2900 fps and have taken numerous zebra, wildebeest, oryx, and those size animals. Zebra are tough, broadside shots on zebra the base of the 250 exits, no matter how much bone it busts in between. I even took a cow cape buffalo with it, base was found far side under the hide. This bullet is amazing, and absolutely enhances every 458 caliber cartridge out there, including the much larger bolt gun 458s........
The other Semi I have on hand is the 458 B&M Super Short. This is a cartridge we designed for the WSSM bolt guns, of course in the Semi it has to be slowed down, pressures in these guns we like at 55000 PSI, in the bolt guns you can run up to 65000 PSI. This is a WSM case, cut, trimmed to 1.640, then squeezed down to .458 Caliber....... In the bolt gun, it runs the same 250 Socom Raptor to 2600 fps, under 65000 PSI. However in the Semi platform, with 16 inch barrels, we have to run the 250 to 2350 fps to stay at or around 55000 PSI.
For that 458 Socom you might want to check out the 250 Hornady as well.... in the 458 SS it runs 2350 too at 54000 PSI......300 Barnes at 2270 fps, and 325 Flex at 2140 fps............. The 458 SS is on a AR 15 Platform.