1. Overview
2. Materials
3. Construction
– Air Gun
– Projectiles
4. Tests
– Test Shot 1: 45 PSI with Wet Tissue
– Test Shot 2: 50 PSI with Metal Projectile #1
– Test Shot 3: 80 PSI with Sabot Metal Projectile (Failed)
– Test Shot 4: Initial Trial Testings with Ballistic Gel at 60 PSI
– Test Shot 5: 25 PSI with Final Ballistic Gel
6. Physics

 

This project aims to produce a airgun capable of shooting a projectile at muzzle energy > 150 Joules (limited by air pump). In comparison, a standard U.S. police department issues their officers a Glock pistol capable of muzzle energy ~ 475 Joules. Although half the muzzle energy of a modern weapon, the shot fired would severely injure anyone upon direct hit.

The details, planning and design closely follows the excellent walkthrough written by Gao Guangyan. All credits and whatsoever goes to him.

Difficulty: 1 2 3 4 5 6 7 8 9 10

Cost: 1 2 3 4 5 6 7 8 9 10

Time: 1 2 3 4 5 6 7 8 9 10

 

1. 1″ PVC Elbow 90 degrees x2
2. 1″ PVC Male Adaptors x2
3. 2″ PVC End Cap
4. 2″ to 1″ PVC Reducing Coupler
5. 2′ of 2″ Pressure-Rated PVC Pipe
6. 3′ of 1″ Pressure-Rated PVC Pipe
7. 1″ Metal Ball Valve
8. High Strength PVC Cement/Glue
9. Car/Motorcycle Tire Valve

 

Before I begin, a big thanks to my Dad for helping me throughout the project, from the tedious hours of sawing and purchasing the materials for me.

The main material chosen for the project is PVC. PVC is relatively cheap, light weight (1/6 the weight of steel), easy to obtain, and are available in many different sizes, making it an ideal construction matteerial. All PVC pipes have a pressure rating, or a ‘schedule’ number. For this project, there are high pressures involved, and only high pressure rated PVC pipes MUST be used. This is important if you do not want the PVC pipes to blow up in your hands, sending PVC shrapnel flying into your eyes and body.

Industry-standard PVC pipes with a labeling of AW(VP) were used. These are considerably thicker than their thinner AE counterparts. They are rated for an approximate 284 PSI of pressure, which gives a three times safety margin over the 80 PSI I will be using.

The design is fairly simple. This image shows the materials in place but they are not yet joined, so I left a small gap in between them.

The larger 2? PVC tube at the bottom stores the compressed air, and the valve is closed. When the valve opens, the compressed air is released through the smaller 1? PVC tube and pushes the projectile out.

So I started gluing everything together…

A projectile stopper is needed somewhere towards the back of the 1″ PVC pipe. The purpose of the projectile stopper is to prevent the projectile from dropping too deep into the airgun.

I drilled a (roughly) 5mm hole straight through the 1? PVC pipe, somewhere close to the end. The projectile will be inserted until it reaches this end, whereby it would not be able to move any further as the projectile stopper is there.

That job was quite fast, as PVC is rather easy to drill.

Then came the difficult part: the projectile stopper itself. My Dad found a twisted piece of rod lying around, so he asked me to use it. It sometimes really helps to have junk all over your house. The rod is internally made of copper I think, and has a approx. diameter of 3~4mm. This sounds deceivingly small, but trust me, that 3~4mm is a pain in the ass to saw.

Once I sawed the small bit of rod off, I managed to get it to fit perfectly into the hole without being too loose or tight.

There wasn’t a need to secure the rod to the pipe because when the pipe is inserted into the joinings, the larger male adaptors at the end of the pipe would cover the hole and prevent the rod from dropping out.

The only component left is the valve to pump the air in. I drilled a hole through the 2? PVC End Cap…

Then fitted the tire valve through:

And finally, I glued everything together.

As you might have noticed, the pipes are bent inwards. I guess, such imperfections do occur.

I decided that, for aesthetic purposes, I gotta make the pipes parallel. I found a spare PVC pipe and sawed off a short length to make a bridge between the two pipes.

Then I taped them up…

And done! The airgun is completed!

Before doing any testings, it is wise to pump the airgun up without any projectiles. It’s a safety precaution to test for any possible leaks and cracks that might be present.I pumped it up in steps of 10 PSI.

Most unfortunately, my airgun has an air leak at the 2″ End Cap above 60 PSI. After more than 3 days of gluing, applying epoxy and cement, I have finally capped up the air leak.

This is how awful it looks now:

It’s a copper patch that I applied all over the leaks. Really difficult to find the exact spots of the holes, so I had to redo and redo and redo…

And it’s finally, finally, completed, with the ugly copper patch covered up.

My original aim was to pump the airgun to the maximum pressure the pump could handle – 100 PSI. It’s a foot pump rated at a 100 PSI limit, but after 80 PSI, even if I exerted all my weight onto the lever, but the piston wouldn’t go in further. As such, due to the pump’s limitation, I couldn’t pump above 80 PSI.

 

Construction – Projectiles

The projectile is probably the most overlooked component. It does not have a definite design, and it’s completely up to you on what material, shape, etc.

For low-damage projectiles, anything from a ball of wet tissue to a hollow rubber leg tip would do.

Rubber Projectile

Here’s the hollow rubber leg tip that I used. It travels at an extremely high velocity with a tight-fit in the barrel. It richochets a lot because it’s rubber, so it’s extremely dangerous indoors. Would be extremely painful if it were to hit a person, but does no physical damage to surroundings.

For high-damage projectiles, metal is recommended. Please take note that by using high-damage projectiles, the impact is capable of smashing concrete, penetrate wood and even kill people.

Metal Projectile #1

I started off with a blunt metal rod, to see what damage it is able to inflict without being sharp-pointed.

I found a rod lying around my house, and but the surface area obviously wouldn’t enable the compressed air to push it out of the airgun. So, I randomly found some rubber stoppers things for doors. My idea was to have the metal rod in a centre of some object that has a large surface area. The rubber stoppers came in different sizes, so it was ideal as I could get one that fit my airgun.

Sadly, despite being able to choose the size, I chose the wrong one =(

It. Just. Wouldn’t. Fit.

So I spent an hour or so sawing, cutting and filing the rubber things until they were are to fit the pipe.

Then I drilled a hole in the middle of each to fit the metal rod through.

And like a satay, it went straight through and was quite secured!

Yeah, it looked horrible, so I taped it up.

I spent a full hour filing the tip of the metal projectile. I ended up with a nice rounded tip. A sharp pointed tip would be better, but more prone to damage. And since I wanted to reuse the projectile, a rounded tip would suffice.

But because the rubber back does not come off the projectile, it stops the projectile from penetrating deeper into the target, with the yellow portion sticking out of the target. As such, I decided to work on another projectile.

Metal Projectile #2 with Sabot

I found a better metal projectile, a nail-shaped rod meant to be used for hammering.

Basically, I had to make the projectile have a tight-fit in the barrel to receive the full kinetic energy of the compressed air, and also be able to travel through the air aerodynamically and finally penetrating the target fully upon impact. Yes, I’m expecting a through-and-through projectile.

For this, a sabot could be used.

“A sabot is a lightweight carrier in which a projectile of a smaller caliber is centered so as to permit firing the projectile within a larger-caliber weapon. The carrier fills the bore of the weapon from which the projectile is fired; it is normally discarded a short distance from the muzzle.”

The design I came up with was this:

The blue rectangles represent the sabot, and the rod shape thing is the projectile of course.

The sabot would wrap around the projectile somewhere towards the middle because the back of the projectile had a square-shape, which wouldn’t allow me to drill a hole that would just fit.

The sabot would fit nicely into the barrel, absorbing the full kinetic energy of the compressed air and accelerate while carrying the projectile with it. When it shoots out of the barrel, air resistance pushes against the sabot and it falls apart with the projectile flying away.

After 4 hours of technical work, I got it done:

 

Test Shot 1: 45 PSI with Wet Tissue

For the 1st test, I used the most readily available projectile — wet tissue. I started off at 45 PSI.

Some parallax reading there because of the position of the camera, but it reads about 45~50 PSI.

I set up a cardboard just outside my house as the target. The distance between the airgun and the cardboard is about 3m.

Because my accuracy is terrible, the wet tissue hit the corner and half of it ended up in my neighbour’s side.

Test Shot 2: 50 PSI with Metal Projectile #1

I loaded the blunt metal rod projectile into my airgun. Pumped it up with a new air pump to 50 PSI.

It was already well into the evening, but I went into my backyard anyway and stood about 3m away from the cardboard target.

And the thickness of the two cardboards which the projectile has to penetrate.

The airgun shot the projectile very nicely, hitting the cardboard almost immediately at an extremely high velocity, caused quite a bit of damage before ricocheting off.

The projectile managed to pierce through two layers of cardboard, which I find, rather incredible already. Chances are, it is able to penetrate through human flesh as well.

This is the front view of the entry point.

Before I concluded for the day, I decided to film another shot.

Here’s it:

 

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There’s a slight recoil, very loud sound, and a hell lot of kinetic energy.

Test Shot 3: 80 PSI with Sabot Metal Projectile (Failed)

This shot was a failed shot.

The airgun was pumped to 80 PSI, which is limited by the air pump.

I test fired the metal projectile with the sabot. It flew out, hit the target, and ricocheted off lightly. Extremely disappointing considering the time I spent to make it. I pumped the airgun up again, shot again, this time closer, but it still didn’t penetrate even the cardboard. It just ricocheted off.

Video:

 

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The projectile simply ricochets off the ballistic gel. It seems that the centre of gravity of the metal projectile is too far behind, thus when it approaches the target, the back swings forward and hits the gel sideways. As I’m unable to find a similar diameter metal projectile with a front C.G., I cannot prove this theory. Also, I’m not willing to redo another sabot as it’s extremely tedious.

However, I managed to prove that the sabot manage to hold the metal projectile tightly in place while accelerating it to high speeds. With a metal projectile that is sharp and correctly-located C.G., I’m pretty sure it’ll do quite some damage.

Test Shot 4: Initial Trial Testings with Ballistic Gel at 60 PSI

Note: This is an extract from the Ballistic Gel report. For more information and details, please visit the Ballistic Gel page.

After I completed my first attempt at creating ballistic gel, I decided to give it a test shot with the pneumatic airgun and the metal projectile #1.

I had a setup for the shot in the, er, let’s just call it attic of my house.

With the small piece of gel mounted on top of a Styrofoam piece, with a cardboard backing, I stood about 3m back. The airgun was pressured to 60 PSI.

And fired!

 

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This is the aftermath of the shot.

The projectile FULLY penetrated the 8.5 cm thick of ballistic gel, plus three layers of hard cardboard.

If you’re thinking that isn’t enough to kill, you’re either Wolverine or a liar.

The metal rod penetrated so deep into the cardboard that it took a lot of trouble to get it out. I pulled hard, and all that came off was the rubber backing.

This is the metal rod with the rubber backing pulled off. You can see the damage caused clearer here.

Test Shot 5: 25 PSI with Thicker Ballistic Gel

Just before my last attempt to conclude the experiment, the air pump had to spoil. The metal lever thingy got twisted, and while I was pumping it, the air pump jumped off the ground and landed a straight clean stab to my foot. So I ended up with this bruise on my feet.

Today I finally bothered to attempt to fix the air pump, so I used hammers, pliers and tools-which-I-don’t-know-what-they’re-called to bend the metal back. In the process, a certain part got loose, and now the air pump’s leaking and wouldn’t go above 25 PSI.

I got the ballistic gel out of the refrigerator as the targets.

Please note that I remade the ballistic gel. The first attempt had too little ballistic gel. This time, however, I was amazingly blur and foolish. For some reason I have still yet to uncover, I used proportions of 14% gelatin. The gel is much harder this time, and feels really tough. Also, the gel is kept constantly chilled as far as possible, and only removed from the refrigerator just prior to shooting. I ended up with a ballistic gel that is TOO tough. I only realised this after the experiment and looking through the pictures, so let’s just make do with it.

As this was merely a small test fire at 25 PSI, I didn’t bother to take videos and pictures. But I got these:

I measured the penetration of the projection, and it’s about 3.2 cm.

Once again, please be reminded that the ballistic gel is supposed to closely represents human flesh, and in fact, it’s much tougher.

After this, I decided that I will not do any further testings because it’ll simply be wasting money to purchase another air pump. I’m setting for the 25 PSI shot and do further estimates from there.

At 25 PSI, it achieves a 3.2 cm. At its full power (limited by the air pump I buy) of 80 PSI, it should be at least 3 times as powerful, thus achieving a penetration of 9.6 cm at least. Of course, this are just estimates, and are definitely by no means, accurate. However, the true figure should linger near that value, possibly less.

To conclude, the airgun, fully pumped to 80 PSI by the air pump, would be able to penetrate at least 9.6 cm of human flesh using the projectile I made. The ballistic gel used here is 14% gelatin instead of 10%, which makes it significantly tougher than the 10% proportion.

If this isn’t enough to kill, I don’t know what is.

 

Formulas from Gao Guangyan

It is possible to calculate the maximum projectile speed attainable with a certain amount of pressure. In this example, we will be using the measurements for the airgun pumped to the maximum 80 PSI (limited by the air pump), and the metal projectile #1 which weighs in at 70g.

100 PSI = 551581 Pa (Pascals)
70 g = 0.07 Kg

Air Tank Dimensions = 2″ diameter X 24″ length = 5.08 cm diameter X 60.96 cm
Barrel Dimensions =: 1″ diameter X 24″length = 2.54 cm diameter X 60.96 cm
Barrel Cross Section = Pi X r² = 0.7854″²

Using Pi * r² * h, we can calculate the volume of the chambers:
Air Tank Volume = 75.3982″³ = 0.001236 m³
Barrel Volume = 18.8496″³ = 0.0003089 m³
Total Volume = 94.2476″³ = 0.001545 m³

When the projectile is at the start of the barrel, the pressure is 80 PSI. However, when the valve opens and the projectile begins to move all the way to the end of the barrel, the pressure drops as there is now more volume for the original compressed air in the air tank.

Since the amount of air does not change, We can use the formula P₁V₁ = P₂V₂
80 PSI X Air Tank Volume = P₂ * Total Volume
Working the equation out, P₂ = 64 PSI, and there is a pressure drop of 16 PSI

Knowing these values, we can calculate the force acting on the projectile throughout the barrel.

The force acting on projectile = Pressure * Area
At 80 PSI (start of barrel), 80 * 0.7854″2 = 62.832 lb = 285.6 N of force
At 80 PSI (end of barrel), 80 * 0.7854″2 = 50.2656 lb = 228 N of force
The average force acting on the projectile in the barrel is therefore (357+285.6)/2 = 256.8 N

When there is force and mass, there is acceleration. And where there’s A LOT of force, there is a lot of acceleration!

Using the formula F=ma,
256.8 = 0.07 X a
a = 3668.57 ms^-2 (That is a lot of acceleration!)

And using the formula s = 0.5at²,
0.6069 m = 0.5 X 3668.57 X t²
Therefore, t = 0.018189 s= Time taken for the projectile to travel out of the barrel

Using all these values, we are thus able to calculate the maximum projectile speed as well as the total kinetic output:

Velocity = Acceleration X Time, Therefore
v = 3668.57 * 0.018189
Velocity = 66.728 ms^-1 = 240.22 km/h!

And because K.E. = 0.5mv²,
Energy = 0.5 * 0.07 * 66.728²
Energy ? 156 Joules!

That is a very high output energy, and with that, I have successfully achieved my target of having a muzzle energy of more than 150 Joules.

Of course with a lighter projectile, faster speeds are attainable. For example, changing the 70g projectile to a 10g projectile would yield the same energy output, but with a projectile speed of 197.5m/s! Reaching supersonic speeds and breaking the speed of sound would be quite impossible with this setup as the speed of sound at 0’C is around 331m/s.

The calculations provide a idea of the maximum theoretical velocity, which is often lower in real life and the calculations do not involve factors such as friction, suck back effect, valve opening time etc. Efficiency of the air gun can be greatly improved like a fast opening valves, or a smooth projectile with little friction. Other factors include the air tank volume, and barrel length. As can be seen from the calculations, the more air tank volume versus the barrel volume, the less pressure drop as the projectile accelerates, and therefore more average force. However, the longer the barrel, the longer the time the projectile will be accelerated and therefore faster speeds.