So you have a decent sized vacant lot of land (be it farm country, woods, tundra, ice shelf) and you think to yourself, "Hmm...I wonder just how far I can hurl stuff, maybe even into the next county." Or maybe you just like the idea of kinetic energies ten times that of a 50 cal bullet shot out of a sniper rifle. Or maybe you want to get into Punkin' Chunkin'. Well, my friends, you just might want a piston-valve pneumatic cannon.
It'll sure beat the pants out of your neighbor's ball valve pneumatic or hairspray combustion cannon.
Update 2014 - We were young and stupid when my friend and I first built this cannon. We made a lot of mistakes and a lot of safety oversights. I highly suggest reading up on modern potato cannon theory and design before delving into a project of this magnitude. Use your head!
Videos of the recent 2in valve:
Three test shots at 40psi, two with paper towel, the last one with a whiffle ball bat. We were gonna move on to damage shots, but it got dark. I should have damage vids up today or tomorrow.
Videos of the old Mk IV 3in valve:
Preparing for launch, I couldn't find the rocket. So I put in a can of lemonade instead. "There's the rocket!" refers to my sudden realization that the rocket was in the barrel the whole time. Rocket, lemonade, and wadding all shot out over 120 m.
My cannon has a record distance of 1900 feet (using Google Earth along with terrain mapping - it happened to land right between two distinct knolls that I could see from GE). That was a 70 psi shot with a 2.5in x 20ft barrel that was so long it needed it's own suspension system.
I have a video of that shot somewhere but I have it find it I'll post it as soon as I do. That was also the shot that broke the cleanout cap.
During the testing phases, the cannon was barely charged halfway, so I could get away with plugging my ears. I make sure to grab hearing protection for anything 65+ psi.
For these two shots we had an audience.
They were thoroughly impressed:
This shot, something caused the piston not to actuate, and we had a dud. They weren't as impressed :-P:
Step 1: Story Time!! (aka theory)
So over the past two summers my friend and I have been working on my brainchild: a 3" porting piston cannon. Porting is the spudding term for the size at the greatest restriction between the pressure chamber and the barrel. The air can only move as much through the barrel as it does through the porting (usually the valve) so if you want more air to hit the projectile, you need a bigger porting.
Piston valves work on a principle similar to a plug in a drain. Pressure on one side of the piston, causes the other side, or sealing surface, to make a seal against the barrel, the porting. This keeps the rest of the air in the chamber from escaping. The region behind the piston, called the pilot, is dumped to ambient pressure, and the pressure difference from the valve causes the piston to get shot back quite forcefully. The porting is now wide open, and the air from the pressure chamber rushes into the barrel, launching the projectile.
Sounds fairly straightforward, right? We thought so. Many people had already build 2" porting valves with little trouble, but I wanted to go bigger. One would think that you could just change the size of the parts from 2" to 3" and voila, you have a bigger valve. Well, yes and no. Moving up to 3" increases the size of the sealing surface by 50% or more, and achieving the right seal can prove frustrating, difficult, and expensive.
So, to save you from some of my mistakes, I present you with two, count 'em, two different designs. One is the 2" porting, which still had quite a kick, and the 3" porting valve I used on my cannon, for those of you with higher levels of patience.
1. Under Load: The valve pressurizes from the pilot end, pushing the piston against the sealing surface. The pressure equalizes between the pilot volume and the chamber. Because the surface area on the back side of the piston is larger than the surface area on the opposing size, the force pushing the valve forward will always be greater.
2. The pilot valve is opened. The pressure behind the piston drops quickly, not allowing the chamber side time to equalize. The force pushing the piston forward drops, while the force pushing the piston back remains nearly constant. The two force equal and then...
3. Actuation. The force pushing the piston back exceeds the force pushing it forward, and the valve opens, allowing the air from the chamber to flow into the barrel.
Step 2: A few words on construction
This part of the instructable is called covering my own ass. :-P I take absolutely no responsibility for whatever happens to whoever uses this instructable. None. Zip. Zilch. Nada.
I haven't worked on this project, or spudguns in general, for a while now. I am not 100% positive this design is optimized. I highly recommend that you read up on valve design and theory and really understand what you are doing before building a piston valve.
Check out http://www.spudfiles.com for all the information you need.
This project involves large volumes of air at high pressure using PVC pipe. PVC was never meant for compressed air technically, though under most circumstances it does fine. This project also involves PVC under load taking large impact stresses, something it isn't terrifically good at to begin with.
I know some of you reading this have been making spudguns for a while and have been mostly lackadaisical about construction materials, I'm sure you've read the warnings "Don't use XXX pipe!" that it could frag and grenade, and have done it anyway with no consequence. I've made non-pressure rated cannons too, and they've worked just fine. However, this is the big league. This stuff can most definitely injure you. If the instructions say to use a particular type of pipe, the only acceptable substitute would be something even stronger.
My cannon worked fine for a while with no problems, but when I decided to improve the seal, the valve worked too well in essence, opened too fast, and smashed the valve. The first time it just broke the fill system, that was an easy fix. The second time snapped the valve in half. This is serious power, people.
Update 2014 - I've come to the conclusion that there really is no 'safe' way to build a PVC cannon. Even using pressure-rated pipe, PVC is designed to handle mostly static pressure, with some water hammer. The forces and shocks involved in any sort of potato cannon, let alone a piston valve, is above and beyond the safe operating limits of PVC pipe and well into the danger zone. I haven't played with cannons in a long time, but I've adopted the philosophy that PVC cannons made from pipe (DWV or otherwise) that experience shock loads ought to be operated in a manner such that if there is a fragmentation, the operator or bystanders won't be injured. That means remotely fired, behind a sturdy wall, dirt berm, etc. I was young and stupid when my friend and I built the first large cannon. Had that valve fragmented at 100 psi and we were standing in our usual spot, we could have easily been seriously injured.
Solvent Welding - PVC is not glued (even though it's referred to as "cement", it's solvent welded, which is where the parts are dissolved and meld together into one solid piece. A good weld should be stronger than the pipe itself, see my failure pictures. If you don't know how to solvent weld PVC properly, I highly suggest taking a course at Home Depot or other hardware store, they're usually free, it will save you a lot of hassle.
**A quick tip on solvent welding: After covering both sides of the joint with cement, push the parts together while simultaneously turning 90 degrees. This ensures the strongest, most airtight joint.**
I don't care what the can of PVC cement says, I think it's a general rule of thumb to give 24 hours cure time before putting joints under pressure. 2-3 hours is plenty if you just want the part workable, for drilling, cutting, and sanding you can start almost immediately.
I tried to give as close to step by step instructions, but since a large portion of these are after the fact, there are going to be minor details left out. This project is a pretty big undertaking, and I'm assuming you have enough sense to try and bridge minor engineering gaps. You will probably have to interpolate a little bit with sizes, just figure out what works for you.
Step 3: Materials
Air compressor of some sort (I would not suggest a bike pump you'll be there a while)
All-purpose or plastic epoxy (JB weld works very well)
Power drill and bits
1 1/4in spade bit
Grinder or grinding drill bit
Saw (with blade for wood, optionally one with smaller teeth for PVC/plywood)
Adjustable Wrenches (2 crescent wrenches work great, wrench and pliers also)
Strap wrench (isn't necessary but will make your life much easier)
Plywood (about 1 sq yd works)
6 feet of 1/4" aircraft cable
Two u-bolt clamps for 1/4" aircraft cable
Medium size rope - 20ish feet
Decking screws - 2 1/2", treated is nice but not necessary
Some kind of deck sealer, optionally you could use pressure treated
Several 3" bolts for mounting the winch (sometimes they come with it)
Fill System (generally 1/4" NPT steel fittings):
Three 2"x1/4" steel nipples
One 1/4" steel Tees
1/4" ball valve (brass or comparable)
Pressure gauge up to 100 or 120 PSI at least, with a way to connect to 1/4" fittings
1in PVC threaded adapter (see step 8)
3in length of 1in PVC pipe
1" electronic sprinkler valve (rain bird)
Source of 24VDC - either use 3 9V batteries in series, or use a transformer/power cube
Extension Cord, and if you're going to use a transformer to power the switch box, splitters *If you have a gas-powered compressor and use batteries in the switch box, you won't need this*
Stereo wire, and lots of other scrap wires
For 2" Valve:
For 3" Valve:
4" Sch 80 PVC Tee (slip x slip x slip)
4" Threaded PVC Female Adapter (slip x thread)
4" Threaded PVC Male Adapter (slip x thread)
3" Female Threaded Adapter (slip x thread)
4" x 3" Reducing Bushing (slip x slip)
3" Elbow (street)
3" Female Adapter (slip x thread)
Step 4: "Guillotine" Style Stand
We got the name for the cannon when we were building the stand - my friend's mom commented it looked like we were building a guillotine. If you have basic carpentry skills and know how to put together wood frames, this should be a cinch for you just by looking at the pictures. If you don't, well there are explanations below.
You will need planks of these sizes (and how we cut them up, if you have the scrap already, you can use that too, save you a trip to the lumber yard):
4x Full length (10ft)
4x 2ft (cut an 8ft into 4)
4x 4ft (cut two 8ft in half)
2x 4ft 1.75" beveled bracing pieces (I did this by finding the length and widthwise center and putting a 45" cut through it, and then beveling the edges)
2x 6.5" (cut from scrap)
Plywood - about 1 ft 9 in wide
Nautical Winch and mounting bolts
20ft of outdoor rope
All 2x4's are joined by decking screws, two or three for each junction.
Put together the short and the tall frames:
The tall one has a 4' plank for the base, two 10' uprights, and two 2' pieces at (not on) the top on either side of the frame. The short one is built the same, except with 4' pieces for the uprights.
Mark the middle of each of the 6.5in boards (you should have a line at 3.25 in running the width of the board). On one of them, drill two 1/4 in holes about half an inch apart, both on the line.
Take the other board and affix the pulley in the middle of the board along the line (so that the AXIS of the pulley is perpendicular to the middle line).
Attach the board with the two holes to the top of the small frame, in the middle of the two 2 ft top pieces. Attach the board with the pulley to the top of the large frame in the same fashion.
This next part is a lot easier if you have a friend. Mark 2' up on each of the uprights, then position the 10ft horizontal with the bottom of each 10' piece on the line. Screw it into place. Repeat for the other side.
Make sure the tall frame is plumb and the horizontal member is level before affixing the bracing pieces. This should be straightforward, you brace it so that the piece is 45" from vertical and the cut ends are mated against the vertical and horizontal members. Like I said, if you have basic carpentry skills, these steps should be redundant.
Cut the plywood into 4 right triangles that are 1ft 9 in along the legs. The hypotenuse isn't that important, but for you math types it would be 17(root)2 or 23.8 in. The easiest way to do this is cut two squares 1 ft 9 in wide, then cut them along the diagonal like a PB&J sandwich.
Use two triangles facing in opposite directions to brace the middle of the tall frame. Space them evenly, leaving an inch or two between them in order to drive screws in. Then brace the outside, making sure to keep the offsets balanced. Use a basic sense of symmetry when placing the triangles; location isn't super critical. Hold each triangle down with a decking screw at each corner, distanced an inch from the corner and angled slightly toward the frame.
Find a spot on the side of the frame about hand level. Place the winch on the frame and trace the mounting holes. Drill the mounting holes and bolt on the winch. Affix the rope to the winch, and run the other end through the pulley.
Step 5: 2" Valve - Design
-3in PVC Tee
-Two 3in Female Adapters
-Two 3in x 2in Reducing Bushings
-3in Female Cleanout Adapter
-3in 90deg elbow
-Two 3.5in long sections of 3in pipe
-6.5in of 2in pipe.
Weld one section of 3in pipe into the middle of the Tee. To that, weld the elbow. If the tee is directional (has a definite direction of flow from middle to outlet) you want the elbow outlet facing the same direction as the tee outlet. Take care to line it up using the injection molding veins on the side of the fitting.
Take one of the 3in x 2in reducing bushing and find the stop on the inside of the 2in part. This should be about 1/4in of raised plastic. Using a grinding bit, grind it down flush with the rest of the inside of the bushing. Finish with some rough sandpaper (there is no need for fine sandpaper)
Step 6: 2" Valve Piston
-3in of 3in diameter PVC rod (www.mcmaster.com)
-Section of 1/8-1/4in neoprene rubber (certain mouse pads will work)
3in pipe is not 3.00in. You would think this would be the case, but no. 3in pipe is really 2.946in pipe. Therefore, PVC rod that is 3.00in in diameter will not fit, especially after the 3in pipe is solvent welded (which causes the pipe to shrink). So if you have a lathe, this part is easy. If you don't, well, better get sanding.
Once you get it to fit nicely, you can take out excess mass by boring a hole in one side of it. I used 1 1/4in because that's the largest spade I had, but you could probably take out up to 2in. The lighter the piston, the faster the actuation.
I also ground down the middle into a nuclear-cooling-tower shape to decrease the surface area that makes contact with the inside of the valve.
Trace around the face of the piston onto the neoprene rubber, and cut it out with an X-acto. On one side of the rubber circle, use the back edge of the X-acto to score the rubber. This will help the epoxy bond to it. You probably don't have to score it as much as I did, I was bored.
Apply epoxy to the piston face. Spread it around so it covers the entire face evenly, then press the rubber, scored side down, onto the face. Make sure it's centered, then let it cure.
Step 7: 3" Valve Main Design
Without further ado, the valve that has taken 4 tries to get to the current iteration, the UberValve Mk V.
-4in Sch 80 PVC Tee
-4in Sch 80 PVC Female Adapter
-3in PVC elbow
-4in x 3in reducing bushing
-3in Female Adapter
-Two 3in x 5in long pieces of PVC pipe
-4in x 6in long piece of PVC pipe
-4in Female Cleanout Adapter (the piece that is threaded ID on one side and equivalent to 4in pipe on the other) this can be substituted for a 4in female adapter and 6in length of 4in pipe.
To the middle section of the Tee, weld the 4in x 3in reducing bushing. To one side of the Tee, from now on referred to as the front of the valve, weld in the 4in female cleanout adapter, or 4in pipe and 4in female adapter. To the back of the valve, weld in the 6in section of 4in pipe, and to that pipe weld the 4in Sch80 female adapter.
Into the reducing bushing weld the 5in section of 3in pipe. Weld onto that the 3in PVC elbow. Make sure that the opening points to the FRONT of the valve and is as straight as possible. Usually there are veins from injection molding that you can use to line them up very straight.
Into the opening of the elbow weld the other 3in pipe section and the 3in female adapter.
Step 8: 3" Valve "Bulkhead"
This is what allows you to modify the inners of the valve, and if need be make adjustments in case things aren't quite right.
-About a foot of 3in PVC pipe
-3in Female Adapter
-4in Male Adapter
-4in x 4in piece of PVC pipe
The trickiest part is finding the right length for the 3in pipe. The goal is to get the end of the pipe right where the middle of the PVC Tee is. If you are using a Female Cleanout Adapter, the pipe should be about 11-13in. If you are using a regular female adapter, the pipe will be about 13-16in.
When you are done, you should have a long piece of 3in pipe inside a 3in female adapter inside a piece of 4in pipe inside a 4in male adapter.
Take the 3in Female adapter. Notice there are about 2-8 veins from the injection molding process on the outside of the fitting. There should also be one or two large protrusions about halfway from the slip end. Your goal is to grind off any of the minor bits up until you reach the major protrusions, leave those in place because they will act as a stop when you insert the fitting into the 4in pipe. You want to be left with at least 2in of smooth plastic on the slip end of the fitting. Each model of fitting is different, so you will have to use some artistic license.
Continue sanding this area until you can fit the fitting inside the 4in pipe. It should be snug but shouldn't get stuck, and shouldn't be very loose either. When you are comfortable with how it fits, weld it into the section of 4in pipe.
After a few minutes it should be set well enough for you to insert the 3in pipe into the slip end of the 3in female adapter.
A few minutes after that, weld the 4in male adapter to the end of the 4in pipe opposite the 3in adapter.
Step 9: 3" piston
3in PVC coupler
4in (outer diameter) circular piece of metal
4in sq or circular piece of neoprene, 1/8 to 1/4 in thick
Foam pool noodle
3in x 1.5in piece of PVC pipe
4in rubber O-ring
Take the PVC coupler and grind off any injection molding veins until you're left with a perfectly smooth coupler. Sand off any bumps left over from the grinding process. Then, sand down the entire part until it will slide smoothly inside 4in pipe. If you have a lathe, well then this is easy, and you can skip the next few steps in lieu of just cutting an O-ring groove 1/2in from the edge of the coupler.
Using a chop saw, cut 1/2in off of the coupler. Take that piece, with the rounded side down, weld in the 1.5in piece of 3in PVC. Try to get it close to flush. An easy way to do this is to prime both parts, glue both parts, put the ring rounded side down on the ground (use newspaper or cardboard this will make a big mess) and push the PVC pipe into it. Hold it in place for 60 seconds, because it will want to jump out of the joint. Then let it sit for 10 minutes.
Take the O-ring and make sure it fits snugly around the 3in pipe that you just assembled. If it's loose but only by a tiny bit, that is fine. With the O-ring still on the part, weld the pipe into the cut end of the coupler. Hold it there for a minute because it might try to jump out on you (the reason this happens sometimes is that PVC fittings are tapered).
If the O-ring is too small to reach the surface of the coupler, or if it's slightly too big, fill the joint with a few layers of electrical tape, and trim off the excess.
Take your 4in disc of metal (you can substitute a very thin 4in grinding wheel with the center hole covered with a piece of material and epoxied shut) and JB weld or epoxy it to the side of the coupler opposite the O-ring. Make sure every gap around the edge is filled, it has to be airtight. If you used a grinding wheel, cover the entire surface with epoxy as well. Wait for the suggested cure time on the epoxy you're using.
Cut the foam noodle about 1.5-2in longer than the coupler. Epoxy one end inside the coupler to the metal disc. Optionally you can fill the gap around the noodle with Great Stuf or other expanding foam.
Trace the edge of the disc on the neoprene, and cut it out. Glue the circle of neoprene to the front of the piston.
Step 10: Building the chamber and barrel
If you search the internet, you can look up programs that will tell you stuff like optimal chamber volume, optimal barrel length, optimal projectile weight, but quite honestly unless you're trying to squeeze every single FPS out of it, there isn't any point in wasting lots of time on "optimal" dimensions.
So I chose a 4in x 10ft chamber and 3in x 10ft barrel. The 3" barrel is nice because it offers a large variety of things you can fire. 10 ft is a good number because, well, PVC is sold in 10 ft sections (duh).
Get together these parts - # is the porting size of your valve
3in x 10ft PVC pipe
4in x 10ft PVC pipe
4in PVC endcap (slip) (pressure rated - it should be like $6 and be somewhat hefty, if it is really light and costs $3, it's drain use ONLY!)
4in x # in reducing coupler (slip x slip)
Two # in male adapters (slip x thread)
3in x 2in reducing coupling (slip x slip) (for 2" valve only)
You'll also have to cut a 4-6in section of 3in pipe (3in valve), or two 4in sections of 2in pipe (2in valve).
Solvent weld the endcap onto the 4in pipe, and on the other end weld the 4in x #in reducing coupling. To the #in end of the reducing coupling, weld in the short 3in or 2 in pipe. To that, weld the appropriate made adapter.
For the barrel, either weld the 3in male adapter onto one end, or reduce it down to 2in, weld in a section of 2in pipe, and the 2in male adapter to that.
Step 11: The fill system and pilot
-Male adapter (4in for the large valve, 3in for the small valve)
-Reducing bushing(s) down to 1in
-1in Sprinkler valve - the most common type is a rainbird, make sure to get one that has threaded fittings. It will most likely be female x female
-1in threaded adapter - male or female depends on what kind of valve you have. If the valve has female threads, obviously you need a male adapter, and vice versa
-2in long piece of 1in pipe
-Three 1/4in x 2in steel nipples
-1/4in steel Tee
Weld the bushings into the large male adapter. Weld in the section of 1in pipe, and then to that weld the threaded adapter that fits your valve. Let that sit for a few minutes.
Drill a 3/8in hole in the thickest part of the 1in adapter (the slip side where the fitting overlaps the 1in pipe). Screw in (use some muscle) a 1/4in nipple. Cover the joint where the threads meet the plastic with JB weld or other epoxy. Let that sit overnight.
In the meantime, take two...erm...nipples (yeah I know it sounds funny but it's what the part is called) and teflon tape each thread. You should always wrap teflon tape in the direction of threading, so if it's a typical lefty-loosey-righty-tighty thread, you want to wrap clockwise (if you're looking straight at the thread). That way, the loose end doesn't catch. Most pressure gauges already have a 1/4" male adapter (I'm going to assume it does). Wrap those threads too.
Screw the pressure gauge into one of the sides of the Tee, and one of the nipples into the center of the Tee. The valve goes onto the nipple, and the other nipple goes onto that (assuming your air compressor threads onto a male 1/4").
Sleep. Go on, do it. Unless you have insta-poxy there will be some cure time here.
Feel better? Kay, good. Moving on.
By now the PVC frankenstein-cleanout should be workable. Wrap the nipple sticking out of the PVC with teflon tape, then screw on the tee. Wrap either the 1in adapter or the inlet port of the sprinkler valve (depending on polarity) in teflon, then thread the valve to the PVC. Make sure you get the direction right! There should be an arrow on the bottom of the valve or on the valve somewhere. The air flow should be going from the 3in or 4in adapter, through the reductions, and to the valve. The exit port should exhaust directly into the air.
Step 12: Electrical
Source of 24V
Safety switch like a key switch (recommended)
This is really just a simple series circuit that provides power to the 24V solenoid, causing the valve to actuate. The polarity of the wires doesn't matter.
Once you have selected an appropriate box for your switching setup, drill holes in the lid for the switch(s). Install the switch(s). If you have a safety switch, wire the two switches in series, not parallel. Parallel just allows the switches to bypass one another. Wire the switches in series with the power supply, then connect either end of a 10-15 ft length of stereo wire to the other terminal of the power supply. Connect the remaining end of the stereo wire to the remaining wire of the switch(s).
Drill a hole in the box for the stereo wire, and a separate one if your power supply is powered by a landline (as opposed to batteries). Run the stereo wire and the power supply wires out of the box, and close it. The leads of the stereo wire will connect to the leads of the sprinkler valve.
Step 13: Putting it all together
Three pieces of wood to space out the chamber and barrel
White Lithium grease (the actual grease kind, not the spray stuff)
Six or Eight decently large hose clamps (size will depend on the dimensions of your cannon)
Chunks of wood (scap 2x4's work well)
Teflon tape the threads on the barrel and chamber. CAREFULLY screw the chamber and barrel onto the valve.
Take two hose clamps, undo them, and then link them together to make one big hose clamp. Repeat this two more times. Place the three or four large hose clamps evenly spaced around the chamber and barrel. Insert the wood and foam as spacers, and tighten down the hose clamps EVENLY. Like a car, don't over tighten the threads! You want the barrel and chamber to be parallel, although angled slightly is acceptable.
Take about six feet of aircraft cable. With an equal amount of cable on each side, loop it around the barrel half of the valve (the bottom), then wrap it around the chamber half. Secure a U-bolt onto the cable as close to the valve as you can.
You might want to prop the cannon up on chairs for this part. Put each end of the aircraft cable through a hole on the top board of the small frame. Choke up on the cable so that the valve hangs about 8 inches below the top of the frame (at least far enough so it can swing back and forth without hitting things). Attach the U-bolt on top of the board. Take a bungee, attach it to the cable between the frame and the valve, wrap it around one side of the frame, and attach it to the cable. Do this for the other side as well.
Tie the end of the rope onto the end of the chamber, right behind the endcap. I usually use a strangle snare or a hangman's noose knot with 4 coils whenever I don't want something going anywhere, but I suppose you could use a basic slipknot, bowline, or tautline hitch if that's what you're more familiar with. As long as it doesn't come loose.
Cover the piston with white lithium grease, then slide it into the valve. Close the valve with the cleanout.
Attach the electrical leads to the sprinkler valve and connect it to your switching device. Test to see that the valve works. You should hear a "clack" when the solenoid engages.
Connect the compressor hose to the fill system. Make sure the valve is open, the compressor is plugged in, etc. Test to make sure the chamber pressurizes.
Step 14: The launch phase
Anyone within 20 feet of the cannon should wear hearing protection. Anyone not crucial to the launch phase should not be within 20 feet of the cannon. The cannon operator should be at least 10 feet from the cannon, and if possible behind something.
Once you have started launch prep, NO ONE should be along the fire line of the cannon. This is the line drawn along the length of the cannon, extending in front of AND BEHIND the cannon. You should preferably have some sort of barrier behind the cannon, like trees, and nothing that you don't want obliterated should be downrange.
Check to make sure nothing important is downrange.
Load the barrel with wadding and ram it down with an 8-9.5 foot pole. Don't ram it to the very back of the barrel. Load your ammo, then crank the cannon up to the firing angle.
Pressurize the chamber. For the first firing, keep it below 50 psi.
Turn off the compressor, loudly announce a warning, or use a bullhorn/airhorn. Make sure once again no one is downrange. Then give the countdown.
Engage the switching mechanism for 3 seconds. This will make sure as little pressure as possible is left after firing.
Go fetch your projectile.