I've omitted the spring, catch, etc. There are just the basic parts - enough to show how it's going to work.
- PVC sheath that fits perfectly around 1 1/4" PVC
- 1 1/4" PVC plunger tube
- PVC bushing cut/made to mate with the plunger tube. A wye shoved into the bushing should help alleviate any dead space concerns.
- Plunger made mostly of thinwall PVC, with a skirt seal at the front. The spring goes inside the plunger.
At rest, the spring will push all parts to the front of the blaster. The blaster will be primed by physically pulling the plunger tube back, and the rear of the plunger will catch the trigger.
Tinkercad, a free online 3D drawing tool. It's certainly not the same as AutoCAD or other software that can be printed out/fed into a machine for making parts, but it's good for showing what you're doing!