I've only recently watched "Bloke On The Range" on youtube. He had 2 videos that were fascinating and relevant to this forum's interest.
1. A springer airgun that replaces the bolt on a swiss rifle. Trigger fires as normal, but the pellet (roundball actually) fires through its own barrel inside the rifle.
https://youtu.be/nc0miOdd03Y
2. Cartridges that chamber and feed normally, but are actually pressurized vessels with a plastic (?) bullet that matches the bore. You charge each cartridge with a pump and the firing pin releases the pressure.
https://youtu.be/XTyv5Z-3JVk
I'd love to see 1 redone with C02 and appropriate sized lead roundballs. For a 22? You could even use 22 pellets. 2 intrigues me even more. Website is in french unfortunately and I'm sure shipping to the states would be not worth it. But I love the idea.