Well there is a 450 Bushmaster with a .500" casehead but a larger rim as well. What about a .440" rim on a 460 S&W case? Yes factory 460 ammo runs hot but at say 40 to 45K psi chucking 250 to 300 grain chunks of lead would be cool IMHO using dies and reamers for the 460. Same concept as the Beowulf to have a plug and play system for a say a M4. One advantage could be more rounds in the mag over a Beowulf.

Has this been done?

Please check your figures again. I believe the 450 Bushmaster has a rebated rim @.473"dia (mine measure .470") as the parent case is the 284Win.

I meant larger than .440" Sorry for not being clear. It could be done with a 470 rim too