410ga brass case reloading bullet size

[danyboy]

Just bought a Boito 410ga 12" barrel Hiker break open shotgun. Barrel slugs at .397" at the muzzle (was tight for about a couple inches at the muzzle then dropped loose). So I assume it's a full choke barrel.
If I were to reload brass shells, what size bullet could I use (would a .41" bullet push up the pressure too high, a .401" too loose for the chamber or should I go for round balls ?

[dondiego]

I doubt if you could get the .41 bullet to chamber and I would worry about pressure issues if you did. You might pose this question on the Casting for Shotgun forum.