Cascade Failure Triggered Double Burst Disk Pneumatic Airgun

Phew! That's a mouthful of a name.
It's been far too long since I've posted an Instructable. But rather than list a bunch of excuses, let's get started on today's project.
My latest airgun build follows a design called a cascade failure double burst disk system.  This is the most powerful and efficient gun I've built yet, and is relatively cheap: the whole thing cost less than 40 USD for me.

If you want to make something more efficient, powerful, and unique than the generic ball-valve, diaphram, or piston gun, try looking into burst disk pneumatics and designs such as this one.

PLEASE do not try this if it is your first cannon/airgun build. I will not be held responsible for injuries involved around building, testing, or shooting this design.  Failure to do research can lead to exploding pieces and severe injury.  This Instructable has been written by an experienced builder for others seeking new designs and challenges, not novices.

Special thanks to Spudfiles for having so much knowledge available. I built this gun after reading on that site for about four days!
Thanks also to jackssmirkingrevenge on Spudfiles for answering my questions about designs and linking me to more info.

Side note: I'm not sure what category this should go into. Anyone have some suggestions?

Quite simply, burst disks are one time use valves, extremely efficient at dumping air. Fast.  It does what it's name suggests - once it reaches a high enough pressure, the flat disk material ruptures and releases the pressurized air behind it. For more specifics, visit the spudfiles wiki: http://www.spudfiles.com/spud_wiki/index.php?title=Burst_disk

The pressure at which a disk ruptures can vary depending on material and diameter.  Aluminum foil is a decent, easy to obtain material and 1/2" disks will be used for this Instructable.  Other things, such as plastic plates, duct tape, plastic sheets, and soda cans can also be used depending on the cannon design. A piece of tape 6mm in diameter can take over 300 PSI without bursting! You'll most likely want to experiment with materials and multiple layers if you create your own cannon design. To see a sample list of disk materials and breaking pressures, visit: http://www.spudfiles.com/spud_wiki/index.php?title=Burst_disk_material

Cascade failure is the use of taking multiple disks and letting them set each other off using a domino effect, taking advantage of pressure differences in a system. Because this is more complicated to explain in text, please view this link for more info and pictures:  http://www.spudfiles.com/spud_wiki/index.php?title=Triggered_burst_disk

I recommend doing some research and skimming through the Instructable before building. It may be a good idea in case you wish to change the design.

In first designing the gun, I drew up the whole design and calculated a chamber volume and a barrel to match.  Because I used the smallest commercially available pressure rated PVC pipe and fittings, this ended up being one of the smallest, most compact air guns I've made. Here are the parts I ended up using:

1 x 1/2" coupling
1 x 1/2" elbow
1 x 1/2" ball valve
1 x Schrader valve (found on bikes and cars)
2 x 1/2" union sockets (important - this is what makes the burst disks usable)
2 x 1/2" four way intersection
4 x 1/2" plug 
2 x 1/2" threaded by 1/4" hose barbs
1 x 1/2" brass nipple
1 x blowgun trigger
2" x 1/2" reducer (Most likely this will not be sold as a part - I combine two reducers to get what I needed)
2 x 2" couplers (I used 3 in the end)
1 x 2" plug
~1' x 1/4" pressure rated hose
2 x 1/2" hose clamps
1 x roll of teflon/pipe threading tape
2" PVC Pipe
1/2" PVC Pipe

I highly recommend using PVC cement and solvent welding your pieces together rather than using epoxy. If you've never solvent welded, check this link: http://www.spudfiles.com/spud_wiki/index.php?title=Solvent_welding

To connect all 1/2" fittings, you need small sections of 1/2" PVC. I just used a miter saw to cut out many 1.5" pieces of 1/2" PVC. The chamber and barrel were also cut with the miter saw.  According to my calculations, a 12" x 2" chamber would require about a 3' x 1/2" barrel, so that's what I started with. (The stock ended up being too long to hold comfortably, so the dimensions were shrunk down to a 28" barrel and matching chamber, hence the third coupler. This didn't seem to compromise power at all.)

Before attaching all of the parts together, every plug needed a hole drilled to accept some unique fitting. This would be nigh impossible after the gun is assembled, so be sure to attach the hose barbs, nipple (and blowgun trigger), and fill valve (Schrader valve) first. Because I always end up tearing a crooked hole in PVC when using large drill bits, I used one size smaller than I needed and carefully widened the hole evenly with a Dremel and file.

I've had trouble attaching fill valves in the past. My ultimate design is to use an extremely fat car valve with epoxy in such a way that it's physically impossible to push the valve out of its hole via pressure. I've had this happen with thinner bike valves and I've strayed away from using them. I didn't want to glue the brass fittings in case I needed to remove them, hence the careful drilling, tapping, and use of teflon tape.

The gun is pretty easy to assemble, just follow the pictures:
Attach all 2" parts to make a chamber. From the back, attach an intersection, union socket, intersection, and union socket. Attach the ball valve and elbow as a separate unit.  Taking apart the back union socket can make assembly easier.  Attach the fill valve plug in the back bottom intersection, with a hose barb plug on the opposite side. Attach the trigger plug assembly in the front bottom intersection.  The other hose barb plug attaches to the free end of the ball valve.  Attach the ball valve unit to the top front intersection.
For simplicity, I usually use a break action loading style, where the barrel is just pushed into the front of the gun.  The union socket was too lose for this purpose, so a coupler was glued in the front. This also gives extra material to grab when trying to attach the barrel or screw/unscrew the union socket.
After finishing this gun, holding, and firing it a few times, I really did not like how long the air tank was and how its length made it unwieldy. I hacked out 4 inches of pipe from the center of the chamber and fit on a 2" coupler. Even though the chamber has a smaller capacity, the fact that the whole chamber is pretty much doubled in wall thickness adds a sense of security when handling it. The new chamber volume corresponds with a 28" x 1/2" barrel, so that's what I use now.

Tips:
- When assembling, be careful to glue the tightened, FINISHED trigger plug in straight. Extra tightening can cause it to be crooked in the final product.
- The elbow and ball valve needed to be glued in offset or else its hose barb would be way too close to the other hose barb.
- I tried to make my intersection parts line up with each other vertically, but the nature of the union socket makes it hard to do so.
- I attached the union sockets so that the outer ring would be captured on the front intersection piece. This would guarantee that I didn't drop a crucial part of my union socket(s) and lose it.
- Put the hosing on last! Because it is not prone to twisting, it can make it hard to place your gun together without it wanting to 'unwind' with the hosing. It's hard to explain, but you'll know if you do it wrong.

After assembling the gun, I took out all of the brass fittings and put threading/teflon tape on them. It doesn't really matter when you do it.  When attaching the hose DON'T FORGET THE HOSE CLAMPS. It's easy to leave them off by accident.

An adjustable wrench makes hex fittings very easy to deal with. I don't suggest using pliers like I used to.

Sadly, because of this design, the hose is right in your line of sight while aiming. I may look into another shape variation, but for now the minimal dead space and cost is pretty ideal.

The gun is finished, or at least the basic design is. But the burst disks are the most crucial part of this design.  Because it's a cascade failure design, it takes twice as many burst disks per shot. Better start making some disks!
There are many factors to consider when picking material, number of disks, and operating pressure. Luckily, if you made my gun design, I did all of the experimenting already! Keep in mind some values may very depending on if your aluminum foil is somehow different.

A 3/4" PVC pipe section is the perfect template for cutting aluminum disks. There is plenty of excess aluminum outside of the union socket's O-Ring when assembled, so it doesn't have to be perfectly center when reloading.  The actual size of the face being exposed to the air is 1/2". Here are rough experimental values I've found:
One disk - 50 PSI
Two disks - ~70PSI
Four disks - ~120 PSI
I'm only certain for one disk because that's the number of disks I use per union socket.  Assuming you read about how cascade failure works (Step 1), you know that the center intersection and main chamber will need to different pressures. The ideal I've found with my gun is to use 40-45 PSI in the front and 80-90 PSI in the back. My pump isn't accurate, so anywhere in this range always works for my gun.
You may consider using two disks per union socket so that you get a higher pressure and more power. I highly recommend not doing this.  The 1/2" PVC fittings can take a max recommended pressure of 150 PSI.  If, say, 70PSI is used in the front, and about double is used in the back, your pieces will be under a lot of strain and near it's max recommended pressure. I haven't (yet) had luck using two disks, plus it takes FOUR DISKS PER SHOT. So unless you love cutting foil circles, just stick with single disks for simplicity and safety.

When making the disks, be very careful not to wrinkle them too much. If there are nicks, pinches, or defects of some kind do not use them. They will explode under much lower pressure and can result in misfires. I store all of my disks in a wide lid box. I trace the circles with a mechanical pencil on a smooth, clean surface. I've had problems with crumbs and dust denting my foil while tracing.

Here are the steps I take every time I want to fire my gun:
1. Open the ball valve.
2. Remove barrel, load ammo but do not reattach.
3. Open front union socket, place burst disk on the SMOOTH side, and attach the O-Ring side on top, twisting the outside shut.
4. Open back union socket, place burst disk on the SMOOTH side, and attach the O-Ring side carefully with the outer threading slowly bringing the two parts together.
5. Begin pressurizing the gun (40-45 PSI) and stop.
6. Close the ball valve.
7. Continue pressurizing the back chamber (80-90 PSI) and stop.
8. Disconnect from fill valve. I also consciously listen for possible air leaks.
9. Holding the front coupler, attach the barrel. Do not hold the union socket or gun body and twist the barrel on, this can unseal it while under pressure!
10. Aim and pull the blowgun trigger.

Yes it is a lengthy process, and after a while it becomes habit. It takes much longer time to prep than a normal air rifle, but the power output is pretty astounding compared to my past designs. Here is a video of me loading the gun:

I didn't like that everytime I opened the back union socket, the gun parts flopped around while attached to the hose. To help with this situation, I made a linear guide that could slide open with the gun parts and make loading the back disk easier and faster.  The tension and inflexibility of the hose makes the front end bend outwards anyways, but it still slides open and closed relatively easy compared to when there is no guide.

This is completely optional and possibly done in too complex of a manner, so if you want to try your own I've included pictures on how I assembled my guide.

Here are some random damage pictures. For the most part, I always use nail dart ammo, but feel free to use anything that fits in the barrel! You may enjoy shooting AA batteries, steel bearings, or sharp projectiles in a sabot: http://en.wikipedia.org/wiki/Sabot#Types_of_sabots

NEW - You can make these upgraded darts here:
https://www.instructables.com/id/Reusable-Water-Resistant-BlowgunAirgun-Darts/

Here's a quick video of the loading process (again) with a dry fire. Skip ahead to 1:30 if you just want to see it shoot: