mtnbkr1,
There are several semi-custom rounds available. The 50 Beowulf is a nearly as powerful (in a rifle) and the 500 Smith is in a revolver, it was made for Ar-15 rifles and uses .500 bullets. There is the 50 GI designed to fit in a custom 1911 pistol. There is also the 50 Wyoming Express being marketed by Freedom Arms (Casull), and of course the round that started it all, the 50 Action Express, available in the Desert Eagle semi-automatic handgun. All of these cartridges use .500 bullets, but I believe the 50 Action Express was the first cartridge to use bullets of this diameter specifically.