I've often wondered if the loooong bullet jump in this situation could be corrected by simply using something like a .444 Marlin case to load the bullet out to the throat. It's not an exact match sizewise, but looks like it would be close enough.
I've read where folks make up brass shotshells for the .410, so the idea might work. It would take a position-insensitive powder, but it seems like it would solve one problem, anyway.
Just something that ocurred to me, should I ever find one of these barrels thrust upon me! :roll: