Sounds like a fun project. Is there a reason you're staying that heavy for an air rifle projectile? It makes sense for the PCP shooters but would seem to be a tough sell to the spring-piston guys. Going with the Keith SWC seems like an unlikely choice but then again, I know next to nothing about making bullets.
You said Corbin dies and after looking them up are we talking about swaging bullets or casting them? When you originally said something about bullet molds I assumed you were talking casting but Corbin looks like a company that deals in swaging bullets. Maybe I'm just confusing things?