I have wanted to make something like this for a long time, but couldn't quite figure it out. Thank you to Scott Scriven - http://toykeeper.net/spud/ - for the idea.
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
This was the eastiest part of the assembly:
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.
Step 3: Build Barrel and Diaphram
1) Whatever you chose for your parts for (C), you need to remove the stop ring on the inside of the 1/2" FS, so the 1/2" pipe will slide through it instead of just fitting on the end. I used a sanding drum on the end of a dremel.
2) Cut the diaphram to just barely fit into the inside of (D) (1 1/2" FT). It should fit freely, but not have a lot of room to slide side-to-side.
3) This is the most critical step in the process. Take the time to understand how the gun works. The inside end of the barrel must be flush with where the diaphram sits on the 1 1/2" FT end of (E). To do this, set the diaphram on the end of (E), holding it upright, and screw it into the chamber until it is snug, then back it off a little. Still holding it upright, slide the barrel in until it sits on top of the diaphram. This is where you want it cemented. Mark the location where barrel enters (E). Remove (E) and the diaphram and then cement the barrel in, making sure to keep the end centered in the chambe where it will meet the diaphram.
Step 4: Build Inlet and Trigger Valves
1) Drill and tap the trigger elbow (F) and the inlet valve end cap (G). You will need a 21/64 drill bit and a 1/8-27 NPT tap.
2) Wrap the threads on both of the tank valves and screw them in.
3) Wrap the 3/4 MT on (F) and (G).
4) I ground off the end rim on the valve extender so that the plunger could be pushed down flush. Then, attach it to the valve on (F).
Step 5: Final Assembly
1) First screw (F) and (G) together.
2) Screw the (F-G) combo into (E) very tightly (so you don't have any slip left when you seat the diaphram).
3) Wrap the 1 1/2 MT of (E). Lay the diaphram in place and screw the (E-F-G) assembly on until it just sits with the diaphram.
4) Before you attach the pump, test it out. I put about 20 lbs into it, shoved some tissue in the barrel and pushed the trigger.
5) Attach the pump and secure it to the chamber. I cut a small length of PVC, indented the ends to match the chamber and the pump, held it in place with a couple of zip ties and wrapped it all with electrical tape to keep it in place.
Step 6: Next Steps
So, what is next?
1) Install a new diaphram. The one shown here seemed to wear out after about 50 firings. Maybe too flexible?
2) Get a new pump, like this:
To make it a cock action gun.
3) Make one out of clear PVC, if I can justify the $$$$.
4) Make on out of brass.
5) Try out some different triggers. I have about 6-7 ideas floating right now.