As much as I enjoy seeing the large pvc builds, or rifle-type copper pipe guns, I decided to go a slightly different route and use a pressure-washer gun and some hydraulic fittings in order to achieve much higher pressures than normal. My inspiration for this build was the following project:
http://www.spudfiles.com/airsoft-pellet ... 7-105.html
Ideally I wanted a 12/24v solenoid valve as the trigger, but finding one that can handle upwards of 1000PSI is hard to say the least- most use weird thread styles or cost as much as a good 2nd hand car. Regardless, I ended up using the pressure washer gun which has a 3/8" BSP inlet, and coupled that with various fittings including a ball valve and a quick-release connector to act as the filling port ( being filled via a PCP airgun pump rated for 3000PSI ).
The cannon was designed with versatility in mind, so as such I can change barrels easily- each one is mounted/mated to a 1/2" BSP fitting with epoxy and grub screws, with a small magnet acting as a de-tent. All projectiles are a very nice fit in their barrels so O-ring de-tents are not necessary, air loss or blow-by is minimal, with a very nice loud crack upon firing.
At present I have 3 barrels, a 7mm brass tube for 16" crossbow bolts, a 19mm aluminium tube, and a 22.5mm aluminium tube; the larger two barrels chamber steel ball bearings, empty Co2 cartridges, and an old Brushless motor rotor/magnet.
Power levels vary depending on ammo type, but with the current set up I can achieve upwards of 60ft/lbs of power and 130fps with my heaviest slug, or 30ft/lbs and 200fps with the lightest. Nothing record breaking but pretty respectable for a very compact cannon- I have just ordered some 1" BSP fittings to build a new air chamber that will hold roughly double the air capacity of the current set up, will do some testing once those arrive in a few days hopefully.
I won't bombard you with too many pictures- I have a project page on my WEBSITE, so here are just a few highlights:
I also have graphs showing recorded weights, velocity and power of various projectiles and differing air pressures- currently I use 1000PSI, but in principle I can go upto 3000PSI... if I was that brave. 1500 is as high as I can go at present as there is a small leak in my cobbled together 'aircylinder', between the T piece and the first male to male adapter- PTFE tape can only do so much it seems. I will have more Dowty seals in my improved set up, and may replace the T with a 90 degree elbow if I can find one with nice large mating surfaces- Dowty seals simply won't work with the more slim-line fittings.
Anyway, this is my Cannon, hopefully you liked it and I will keep this thread updated as I make improvements. Cheers.
), so hopefully I'll be able to do something sooner or later at any rate- this little monster demands shooting! 
