you load one of this bullets in front, it probably sticks when you press it in. than set a primer on the nippel and fill the powder load into the chamber. obviously the outside hammer must cock an internal firing pin, maybe an internal hammer who hits on a firing pin. I don't believe an internal hammer hits the primer because it would be to complicated to seal the chamber if a hammer move into this. a hole for a firing pin is much easier.
I believe the reason for this bullets with groove is you lost a good part of the powder energy when the bullet have to be forced into the rifling of the barrel.

think we agree that this is a first class gun