
That's the gun(?) stock, compared to a nitefinder

Step one: Pop out the internals after unscrewing the gun.

Step two: Cut the barrel around that mark, and wrap E-tape around it until a PVC coupler can fit snugly on it.

Goop the coupler on, and stick on a CPVC coupler, which fits perfectly

Lube everything up and flare out the plunger head and you're done!
You need to glue the CPVC coupler on, otherwise it will fly off after a few shots.
Ranges: I really can't tell, it is extremely difficult to make it go flat, but I know that it at least goes 60 feet flat with rear loading a CTD in a 5 inch acrylic barrel.