Building a marble shooting airgun

Air guns are normally small air powered rifles that shoot relatively small projectiles, primarily used for target practice. Usually, they are pumped up with an internal cylinder, activated by hinging the barrel towards the stock.

My home made air guns experiments however take their lineage more from potato gun technology than target practice rifles.

It all started finding a small cheap air compressor at a yard sale. I was looking for something to do with this air compressor, but with only 1/6'th horsepower, it was too wimpy to consider getting air tools. I experimented with "bottle rockets" - sort of.

Next it occurred to me that I might try to shoot a marble out of a piece of copper plumbing pipe. Except that the commonly available gauges of marbles are too large to fit inside 1/2" copper plumbing pile. I took a marble to the hardware store and tried it on various types of pipe they had. Plastic electrical conduit turned out to have just the right inside diameter. So I bought some of that, some 1/2" copper elbows, a ball valve, and some 1.5" ABS pipe and end caps to make a pressure vessel. I was also fortunate to find an adapter to go from the 1.5" ABS pipe to screw thread, and then to 1/2" copper pipe. All in all, the parts cost me around $25 Canadian. Very cheap.

I put all this together, with an innertube valve stuck in the ABS end cap to pump this up. I wrapped electrical tape around the ball valve's thread to bring it to the size of the conduit, and jammed the conduit on top of that. The barrel side is only pressured for a few milliseconds before the marble is pushed out of it, so the joint didn't need to be a perfect seal. I jammed a marble with some paper towel behind it to cut down on blowby, pumped up the pressure container, aimed at my wooden fence, and snapped the valve open with my fingers. I positioned myself such that the ricochet of the marble wouldn't hit me, but was ready to duck all the same. But there was no bounce. The marble had become embedded in one of the boards!

At this point, I got excited. So far, I had just done this out of boredom, but now I really wanted to build an air gun!

Next I made a piece of wood to hold the conduit and pressure vessel together. This was also important to hold the conduit straight - the plastic conduit is very soft, and it had a bit of a curve to it, and would even sag from gravity. I also built a spring loaded valve opening mechanism. Then I made a stock for it, so the whole thing could be held more like an actual gun.


    Spring loaded valve opening mechanism
The mechanism essentially consists of a coiled spring, with a string tied to the valve to pull it open. A hook holds the spring tensioned. When the trigger is pulled, the hook lets go of the spring, and it snaps open the valve. Its important that the spring can open the valve very rapidly. If you can see the motion of the valve opening, its opening far too slowly. The main flaw with my first design was that the trigger was too far forward, which made it awkward to hold the gun.

Performance

I found that it could easily put a marble through 1/4" plywood. With 1/2" plywood, I had to make sure the compressor was at 100 PSI for the marble to go through. I was quite pleased with myself that I was able to put a marble through 1/2" plywood. But try as I might, I never managed to put one through 3/4" plywood.

Next I measured the speed. I set up a digital camera to record video half way between the gun muzzle, a piece of wood I shot against. Then I extracted the audio from the AVI files with Virtualdub, and used Audacity to measure how much time between the pop from the marble coming out the muzzle, and the bang when it hit the target. I found I could consistently get between 120 and 125 meters per second. This is good - any air gun that shoots faster than 152 meter per second is considered a "firearm" under Canadian firearms law and needs the owner to hold a firearms license, and register it as a firearm. There's no limit on projectile weight or kinetic energy as long as the speed is below 152 meters per second. I did exceed that speed with my ABS pipe cannon though. Would be funny to try to register that!

Accuracy

I was initially surprised with how accurate this thing actually shot, considering that it is essentially a musket. At about 8 meters, the spread pattern was all within a 20 cm circle. Not too bad, but I wanted to do better.

Marble rifle

My second marble gun was designed to be more powerful, more accurate, and easier to load. I used 3/4" elbows to allow more airflow to the barrel, a bigger ball valve, and a much more accurate aluminium pipe as a barrel.

I couldn't find a valve that size that turned really easily, so I needed to use a much stronger spring for pulling the valve open. I also added a "cocking lever", which when pulled back, tensions the spring, and closes the valve (these had to be done separately on the first gun). With the more powerful spring, pulling the spring by hand would be very painful to do repeatedly. I also made sure to arrange the mechanism such that the trigger would be in a more natural position.

    Spring loaded valve opening mechanism, and cocking lever

The mechanism works effectively, but its not beautiful. The much larger forces involved also make it a little less elegant. Even with the more powerful spring, it takes about 15 ms for the valve to open (measured it) - which is already a bit on the slow side. 15 ms is about as long as it takes the marble to travel the length of the barrel.

I also used a bigger air chamber, although that probably doesn't help much, because by the time the marble leaves the barrel, the air has only expanded by a fraction, so there wouldn't be that much pressure drop. It primarily means it takes more air to fill, and that the gun is louder, because there is more air that blows out the barrel after the marble leaves. The noise is comparable to that of a regular .22 caliber gun.

Rifling the barrel

I also rifled the barrel. Rifling the barrel gives the projectile a spin along the axis of travel. This in turn stabilizes it, as it prevents the projectile from getting any sort of transverse spin (which it always does otherwise). The combination of a transverse spin, and the Magnus effect - which causes the projectile to be pushed in an arbitrary direction, is what makes muskets notoriously inaccurate at a distance.

However, proper rifling would require very complicated machining equipment - something I didn't have. I settled for helical scratches, which guides the wadding to spin, and in turn spins the marble. The marble itself is surrounded by wadding on all sides except for the front.

The spiral motion was accomplished by putting a wooden pulley on the end of the barrel, with some string around it. I used a wooden stick with a screw in the end of it to make the helical scratches. By pulling the stick through the barrel, while holding the string that went around the pulley while I moved the stick through the barrel, the barrel rotated at a constant rate with respect to linear movement through the barrel, so that all the scratches ended up with the same helical shape.

When I push the marble and wadding down the barrel with the ram-rod, the ram rod always turns as it goes down, which tells me that the rifling does in fact turn the wadding and projectile, at least it does on the way in.


Above: Wood block
glued to wadding

Right: Marble and wadding
inserted in barrel, before
ramrod

However, initial results were disappointing. Accuracy was no better than my first smooth bore musket style air gun. From that, I figured the main other source of inaccuracy was that when the marble left the barrel and separated from the wadding wrapped around the back of it, that it somehow gets a slight random sideways push, which would throw it off a little bit. So I started experimenting with wadding technique - bigger wadding, smaller wadding, and more carefully wrapping the marble. But accuracy didn't improve. I finally had the idea that somehow the wadding slipped sideways past the marble. So I tried gluing a small square of wood to the centre of a piece of wadding, which would force the wadding to stay centered behind the marble as it traveled down the barrel. I made the blocks out of light wood (cedar), and left them square, so only the corners touched the barrel. I was concerned that the wadding might rip off the block, but this was actually not a problem.

Final accuracy

    Impact pattern from 14.5 meters away
With the improvement in wadding technique, accuracy greatly improved. At a distance of 14.5 meters, the total size of the spread was 11 cm side to side. However, there was a bit of aiming error. I actually aimed at the flower above the kid on the box, not the kid. So the sight probably needs a bit of adjustment. Still, this is more accurate than most people can shoot holding a rifle by hand. I used a pillow to support the gun for those test shots.

But the big improvement in accuracy came with a change in wadding technique, not the rifling, so I'm not sure if the rifling made any difference. It could be that 14.5 meters is not enough distance for the marble to really start tumbling through the air. I did some more experiments to try to establish whether the part of the marble that was facing forward in the barrel was still facing forward at impact, but all my experiments suggested that it did not, which would suggest the rifling was not doing what its supposed to do.

Other nonsense to do with an air gun

The nice thing with an air gun like this of course is that it can be used to shoot all kinds of stuff. AA batteries, for example. Or Pens, or sticks of wood. Or filling the barrel with water.

One Idea I got relatively late was to try to shoot the big chocolate covered peanut M&M's. They are about the shape and size of a marble, but not as heavy, and not as round. I wasn't expecting to put one through 1/2" plywood, but I figured with 1/4" plywood, I'd have a chance.

With the first piece of 1/4" plywood I tried I didn't have much luck. I managed to break the surface of the plywood just once, but the hole didn't go all the way through. The other impacts left nice little halos from the coloured shell of the M&M. I thought this was kind of neat for marking where you shot to - colour coded projectiles. Shooting the M&M's against a rougher surface like a tree, or my house, All I got was a nondescript little welt of peanut/chocolate crud stuck to the surface, with no discernible colour.

The next target I tried was just some cheap 1/4" underlay plywood, rather than the good quality material I'd used before. With this, I managed to put the M&M through it on the first try.


Blue M&M went in here...

...And came out here

Update

(Jan 2005)
I found a way to make the ball valves turn much easier. Take the handle off, and turn it with a drill until it gets warm. This sufficiently wears down the plastic that it then turns easily once its cold. After that, it did leak ever so slightly, but greasing it stopped the leakage. The reduced force needed to turn the valve allowed me to use a shorter lever, so my spring can open it faster. The time it takes the valve to open is critical.

I also bought one of those 12 volt "Tire inflator" mini compressors that run off a cigarette lighter. The one I bought cost $20, and supposedly goes up to 275 PSI. I have measured the pressure in the tank at as high as 240 PSI. I also reduced the size of tank on my second airgun a little, so it wouldn't take as long to pump up with the mini compressor. The gun will now put a marble through 3/4" (18 mm) good quality plywood. It also makes quite a bang when it goes off.

I also bought some 5/8" steel ball bearing balls, which are the same size as the marbles, but weigh three times as much (about 15 grams). I can put one of those through 30 mm of particle board, or sometimes sideways through a spruce 2x4 stud (1.5" thick).

(May 2007)
I found another use for the airgun. Its a Crazy but effective way to clear a drain

Final words

If you want to build your own air gun, I hope this page serves as an inspiration. However, if this page is not enough for you to figure out how to build one, its probably best if you don't try. A device like that is inherently unsafe, and if you don't know how to design one, you may not be able to anticipate ways that it may fail on you. Something that puts a hole in 3/4" plywood may also put a hole in somebody's head. Pease don't ask for plans - I didn't draw any, and for liability reasons, it would probably be unwise for me to provide plans.

I'm not a gun advocate, and I see no reason why somebody living in the city should have anything more than an airgun. Although growing up on a farm, a shotgun came in handy here and there.

I also have a more detailed write up on this airgun

See also: Marble shooting crossbow

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