I try to understand how it work because never see this. It seems this rifle having an extra nipple so you can load it as a muzzle loader having a fired rimfire case in the chamber with a hole that opturate the breech end. because you have a center fire case here the powder don't move trough the small flash hole into the primer pocket and you have a gap ( the primer pocket) which makes it hard for the flame to reach and ignite the powder. I would open up the flash hole in the case from +/- 2 mm to 4 mm. Than take an used boxer primer, remove the anvil, set it into case and drill a little flash hole into the cap.
This way the powder can trickle down to the cap and come close like it was a rimfire case.

please show pics how this action works. I think it must be a nice toy.
loaded like a muzzle loader you get more powder into the barrel but would it be a good idea to duplicate a 45/70?