Note to readers: This is a transcription of a blog I posted in 2014, following the design and construction of my "Large Bore Cannon".

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Since I started building cannons a kid I've moved towards smaller and more portable designs. I decided to buck that trend with this one. My goal this time is to build a cannon that's a little scary to shoot. It's also been several years, so I'm curious to see what I'll be able to do with my new and improved engineering skills.

This is my design. A large air cannon with a 4″ bore and all aluminum construction. I’m designing it to have a form factor similar to a Gustav 84mm recoilless rifle. The internal mechanism will be similar to my LAW II air cannon. I want this to be the successor to that air cannon, so I’m making it bigger, better and more precise. With it’s 4″ bore, the kick should be pretty tremendous. I’m also adding a quick exhaust valve as a first stage, which should get the open time way down, so it goes off with more *bang* and less *whump*.

Here’s how it works. The piston, which is highlighted in red can slide freely back and forth. The space that the piston goes through, as well as the space behind it is pressurized. Suction from the barrel holds the piston in the forward position. The valve attached to the back is a quick exhaust valve. When this valve is opened, that space behind the piston is vented, this sucks the piston back and fires the cannon.

I’m going to build the the working part of the cannon first. The stock, grip, etc can come later. Here's my initial parts list.

I’m going to get what I can from Metal Supermarket (there’s one near me). The rest I’ll order from McMaster.






Materials:
1. 4″ id aluminum pipe, 1/4″ wall
2. 7/8″ thick by 4.25″ diameter aluminum disk
3. 7/8″ thick by 4.25″ diameter aluminum disk
4. 1/4″ npt stainless hex nipple
5. Quick Exhaust Valve (McMaster #6646K34)
6. 3/8″ Thick Rubber Bumper
7. 5/8″ thick by 4.25″ diameter aluminum disk
8. 1″ diameter by 1″ long aluminum rod
9. 1/2″ diameter by 15″ long aluminum rod
10. 1.75″ diameter by 1.5″ long aluminum rod
11. 3/8″ by 1/4″ aluminum bar 12″ long
12. 24x #10-32 5/8″ long stainless buttonhead screws (not shown)
13. 4x #8 7/16″ long stainless panhead screws (not shown)
14. 1+3/8″ by 1/8″ o-ring (not shown)
15. 4″ by 1/8″ o-ring (not shown)
16. Schrader Valve (not shown)
17. 1/4″ npt to 1/4″ Push-To-Connect Fitting (not shown)

Ok, I got all aluminum stock I need. It was only 70 bucks too! I was expecting a couple hundred. The 4″ pipe alone would’ve been $272 from McMaster.

The only problem: they only had 4″ od aluminum tube, and only 1/8″ wall. For $40 vs $270 though, it’s worth changing the design a little. After looking at it in person, I think the 1/8th wall will be plenty strong enough. I'm only going to charge it to 150psi.