WB
I read this explaination of the .440 CB demise
Friday, January 10, 2025, 09:27

it was a LOT hotter than the .429 DE, about 300 fps hotter.

"With its deeper penetrating performance potential, more common (and less expensive) bullets for hand loaders and Magnum research offering Desert Eagles and accessory barrels chambered for it, the .440 Cor-Bon should have been a smashing success. But here we are, two decades later and this cartridge is a nearly forgotten footnote in American ammunition history. What happened? In short, the cartridge didn't run reliably in the Desert Eagle semi-automatic. Unlike straight-wall revolver rounds, which headspace from the cartridge rim, the .440 Cor-Bon head spaces from the shoulder, meaning the ledge where the narrower neck meets the wider portion of the cartridge case. Cor-Bon had difficulty creating and holding the shoulder position on necked down .50 AE brass. These variations in external dimensions translated into varying head space which in turn resulted in poor reliability. It wasn’t long before Magnum Research discontinued their .440 Cor-Bon collaboration."