A martini is 1" - 14 TPI, 55 degree thread. And if you cleaned up the chamber design of 577-450, I'm quite sure you would have something nice, but it would for sure require a custom reamer.
Another option is - remember - 500/450 #2 Musket, my favourite. Much more reasonable, and it was both a target and hunting round of considerable repute in the "good old days"... In order to make sure of a good brass supply, you might consider using the .50-90 or -110 as a base.