Here was my goal:
2) Air powered (relatively safe)
Step 1: Plan and gather parts
A. ~ 2' of 1/2" pvc (schd 40 or 80).
B. ~ 1' of 1 1/2" pvc (schd 40 or 80).
C. Some combination of fittings to go from 1/2" FS (female slip) to 1 1/2" FS. I picked up a few different combination of fittings, each of them producing a little different look. For this gun used a 1/2"FS x 1 1/2"MS and a 1 1/2" FSxFS.
D. 1 1/2" FS x 1 1/2" FT (female threaded).
E. Some combo of pieces to go 1 1/2" MT x 3/4" FT. I found a single fitting for this, but you may have to use multiple.
F. 3/4" MT x 3/4" FT elbow.
G. 3/4" MT end cap.
H. Two tank valves.
I. What they call a "Valve extender". This fits onto the tank valves and has a plunger to release the air. I have some other ideas on what else could replace this, but this is the most straight forward method.
J. Some semi-flexible rubber (or maybe plastic) material for the diaphram. I used some sheet gasket material.
K. Teflon pipe tape (not shown).
L. PVC cement (clear is best, to keep it looking clean).
You will need a drill and a tap set also. To thread for the tank valve, you need a 21/64 drill bit and 1/8-27 NPT tap head.
Step 2: Build firing chamber
1) Cut (B) to the desired length. I went for about 8".
2) Cement (C) to (B). Cement (D) to the other end of the (B-C) assembly.