As I'd noted elsewhere, at the shot barrel, Snider brass would work, although a bit short. Magtech 24ga brass could work too if you can get it.

Graf's has the magtech brass


50 Alaskan brass might work for the rifle barrel, so 348 Winchester could be used as a parent case.

Graf's has it


Furthermore, if you find that this gun would end up being more trouble to load for than you would be comfortable with, I'd like a chance to buy it from you.
Consider me if you want to pass it on