If you are a firearm enthusiast, or someone who is seriously interested in ballistics, you would be amiss if you didn’t at least look at a potato cannon, commonly known as a spud gun. While these are not firearms in most jurisdictions in the US, they can offer some insight into how firearms work and behave. They are budget friendly to build and shoot, and can be a lot of fun.
So what is a spud gun?
Well to put is simply they are homemade contraptions that launch potatoes, or just about anything down a smooth PVC pipe. They may use either compressed gas, or a combustion of air and fuel to launch the projectile. Think of T-Shirt Cannons used at sporting events.
For the most part we will focus on the combustion type spud gun, as it is the most similar in operation to a modern firearm. They are comprised of 5 main components, a Barrel, a Combustion Chamber, Ignition Source, Fuel and a Projectile. The simplest cannons use a BBQ lighter, for an igniter and hair spray for a fuel.
Schedule 40 PVC is by far the most used plastic in these contraptions as it is inexpensive, readily available and typically is pressure rated. ABS pipe is another popular build material as it is readily available and inexpensive. However ABS pipe is typically not pressure rated so is not considered safe for use in spud gun construction.
(Really we are using PVC in a way that it wasn’t intended to be used, so despite it being pressure rated, it’s not really warranted against failure.)
What can we learn from a Spud Gun?
We are not going to become better shots playing around with our spud gun. They are not known for their accuracy, think more of a mini artillery piece. However they do serve as good analogs to firearms. Illustrated as follows;
A fuel air mixture is ignited and burns to create hot gases. These hot gases create pressure pushing a projectile down the barrel. The projectile leaves the barrel at a velocity, determined by the pressure of the expanding hot gas, the length of the barrel, and the friction/weight of the projectile. As the projectile travels through the air, (typically subsonic) it is subject to drag and gravity just as a bullet is.
Replace fuel air mixture with powder and it sounds pretty similar to how a firearm works. The pressures are much lower, which means we can use equipment that is much cheaper to measure pressure. Many of the variables such as temperature, barrel length, and fuel type will have bearing on the performance of the cannon. The data analysis skills we may use when evaluating pressures at 50,000 PSI are going to be exactly the same as the skills we use to evaluate pressures at 150 PSI.
When I was a teenager building spud guns for the first time, I had limited resources and limited knowledge. Given my life experiences and the opportunities I have had since that time, I am anxious to be able to take what I have learned and apply it to what got me started.
What work has been done before?
As I mentioned in the introductory article when I started building these cannons there was very little in the way of internet resources. There was no YouTube, and there was little in the way of forums or internet publications that I had access to. Fast forward about five years and I revisited the hobby and found there was a bevy of resources on the internet. Including forums dedicated to the building and shooting of Spud Guns.
This was circa 2003-2006 time frame. (It seems like it was just yesterday). Most of the websites I frequented then seemed to have died off, or are not near as active as they once were but they are still online. So they can still provide a resource.
Spud Tech used to have a very active Forum. It was of the very first forums I ever joined. However it seems like it’s now dead. With the most recent posts in the 2013-2014 time frame. I have no idea if his business is still active or not, they used to offer rifled PVC barrels, which would be awesome to get ahold of today.
Burnt Lakte did some very good studies on Combustion Spud Guns, and published the results and write up on their site. This site is still active and is still a very good resource. Their research is just as pertinent today as when they published it.
The Safety and Legality of Spud Guns
It’s a important to note a few things. First, Spud Guns are not considered firearms by the BATF. With that said there are localities that have outlawed them. Check out the laws before you construct your own.
The second thing to note is that spud guns should be treated as firearms. An average potato weighs 5.3 ounces which is 2,318 grains. If said potato leaves the muzzle at 300fps the energy it contains is ~463ftlbs. By comparison a 45 ACP shooting a 230gr bullet at 950fps contains 461ftlbs. As such potatoes launched from a spud gun do have enough energy to seriously injure or kill someone. Thus all firearm safety rules apply.
The last important thing to note is that we are using PVC in a way that it’s not really designed to be used. Yes, it is pressure rated and the pressures we typically deal with seem to be less then the pressure rating. However PVC can degrade over time, especially in the presence of certain chemicals, when left in sunlight, or if subjected to repeated shocks. There have been documented cases where spud guns have failed catastrophically injuring the shooter.
The Construction of Our Spud Gun
The Spud Gun will will used for testing is going to be fairly representative of guns that I build in my youth. The Combustion chamber will be made from a 4in piece of PVC 18in long, and the 2in barrel will be 72in long, or 6ft. Instead of a muzzle loading design we will use a breech loading design that. This enables us to load the projectile without having to use a ramrod, and permits the use of burst disks.
The breech loading design requires us to do a little lathe work to trim out the inside of the 2×4″ bushing we use. This will allow us to pass the barrel through the bushing.
When PVC glue is a solvent weld. Meaning the glue actually liquifies the surface of the plastic, and then allows the two pieces to fuse together. Similar to how welding metal works. If done properly, the weld is strong or stronger then the rest of the construction. With that said, it is important to start with clean pipe, or use a primer to help clean the pipe prior to the application of the glue. It is also important to wait the full 24hrs.
I can actually speak from experience on waiting the full 24hrs. When I was a teen, built the whole thing in a single day, and I could not wait to fire it off. I did wait 4-6hrs and then took it out on the back porch and “Boom!”. The gun came apart at the joints. I wasn’t injured, shaken but fine. The vapors from the glue are acetone, and they are highly flammable. It’s best to clear them out and start with a clean chamber and use the fuel of your choice.
Where Do We Go from Here?
With the shell of the gun complete, we need to build three more components. We need a way to meter our fuel of choice, we need to mix it for reliable ignition, and we need to ignite it somehow. These are pretty simple problems to overcome but we will do an individual write up for each of these and discuss the way we’ll achieve this for our gun.
As previously mentioned we will be measuring pressure, and velocity. We will also be discussing how we will do that in an upcoming write up. However if you are going to build your own cannon, you don’t need to worry about that.