This is a nightfinder mod because the plunger and trigger are from a nightfinder, and still in the original case (although modified).
It also uses a clip that loads a breech.
This is a picture of the shells (sized for micros, both stock and stefan) and the unfinished breech.
The shells are 1/2" cpvc inside 3/4" cpvc. The breech is made of 1" cpvc.

To the front breech I added a clip, with unknown capacity (I haven't made enough shells to fill it yet).
There is a foam seal to prevent air leakage between the plunger and shell.

The back breech is where the shells are ejected, by this little button here:

The gun mod is just an experiment, not a full product.

The gun is about 2 times the weight of a nightfinder, but it is used with two hands so it doesn't seem heavier.
Being my first major modification, it has flaws, but they are being worked on. The gun is not semi-auto of course. I slide the inner barrel mostly out of the gun to allow a shell to fall into the breech. The barrel is pushed back until the back of the shell touches the plunger seal. The plunger is pulled back, gun is fired. I press the button to eject the shell, and reload another.
The reload time is actually quite fast. I can load, fire, and eject a shell every 5 seconds.
The range is about 55-65 feet fired flat. I thinks its pretty good for my first major mod.
If you want a gun that has accuracy, ROF, is practical, then mod an AT3K.

Doing this with a homemade plunger and trigger could make an excellent weapon. I am already making my own trigger and plunger, and the gun will be reattached to that. Then it will be a homemade. Until then it is a mod.