.410 is around 12 mm. Most garden guns were 6mm., 7mm. and 9mm. rimfire. 9mm. ammo is still available here in Europe with Fiocci and Hirtenberger being the main makers. I think the Eley stuff is now made for them by Fiocchi and some old French Gevelot can sometimes be picked up. Are you sure it is centrefire? Can't say I have ever come across 10mm. before either. If it is 10mm. centrefire maybe some pistol cases could be adapted to fit. Lagopus.....