That picture was taken right after I got all the stuff put on, so it's not quite "finished".
This gun has a 24in long 4in diameter cumbustion chamber. The fuel is metered propane, but I am going to be using MAPP in the spring. The ignition is dual sparks from a 600K volt stun gun (gutted, and placed in a case for easier access to batteries). Two bolts are run through the chamber (one about 1/3 down the chamber, the other 2/3), and there is a piece of 1/2in PVC I cut down the center on the inside of the chamber, and in that I mounted 2 more bolts that are connected with a wire (kinda like a spark strip).
The barrel I use is 2in by about 5 foot long. This is used for spuds and golf balls, with a sabot. For a sabot I use either paper towel, or caps off of aerosole cans. (Since I have Banjo fittings for the barrel now, I can use burst discs.)
Upgrades I have done include Banjo fittings to remove the barrel faster. I replaced the end cap with a 4inch male fitting, with a 2inch ball valve. I am adding a chamber fan to it, which has a timer circuit connected so it will run about 20 seconds by pressing a button. The propane meter is mounted a lot nicer now, no bungee cords. I converted the gun to an over/under, since it was too long. That was done by adding a 3inch street and 3inch 90degree with a male on one end and a female on the other.
I like it, a lot! 
