How long is the chamber?

I've used .444Marlin cases in my .410, loading 12gr. Unique, a thin card, then .410 plastic wad, cut off just a tich short of the mouth, then fill with shot - 1/2oz. then gas check cup -down on shot, then crimped in a .444 Seating die. A .44 mag seating die would also work, adjusted correctly.

The patterns were better than factory 3" or 2 1/2" which I usually used in it.

The same load or type of load might work just fine in a straightened and trimmed .303 or .30/40 case.