We found an effective way to keep ourselves busy and have a ton of fun up at Beaver Island was to have a project we could work on. We discovered this quite accidentally with Protar, so we quite intentionally came up with a plan for this trip. We decided to build a spudgun, of sorts. It’s not your typical spudgun, which utilizes hairspray, propane, or some other explosive gas/vapor as a propellant. We decided to go pneumatic. This meant our gun would have a barrel, connected to a valve (as high-speed as possible), which loops back to a compression chamber. We also opted to fire golf balls, rather than potatoes. The simple design allows us to fire pretty much anything that fits down the barrel. To arm and fire the gun, you first load the projectile into the barrel (it’s muzzle-loaded). You then verify that the valve is closed, and then pressurize the chamber with an air compressor. When you’ve reached your desired psi, disconnect from the air compressor, aim, and rip the valve open as quickly as possible. Voila!

To begin building the spudgun, we raided Jeff’s parents’ hardware store for the necessary supplies. Among these were a valve stem (like you would normally see on a car or bicycle tire), various chunks of Schedule-40 PVC pipe, fittings, end cap, etc., PVC cement, a valve, spray foam, and JB Weld. Since we didn’t know what the hardware would have for piping and valves before we got there, we determined what we could use on the spot.

Here, Tom and Jeff are discussing this matter, while Tom holds some of our other supplies and Reid looks for parts we think we need.

After we had determined what was available and picked out what we needed, Tom helps illustrate how it will all go together.

Once we had all of our materials, we went to the basement of the hardware store, where they keep some a few tools we could use. We needed to cut the long sections of PVC to the right length for the barrel and the compression chamber. At first, we considered using a compression chamber as long as possible. However, we researched it a bit, and found that found that a shorter chamber would be more beneficial to us.

Jeff and Tom are doing some test fitting and determining exactly what lengths we should go for.

A nice DeWalt chopsaw in the basement made quick work of our cuts.

Tom shows the test fitting after we’ve cut our sections.

Once we had all of our pieces, we went back to the cottage to assemble them. At this point, I JB Welded the valve stem in an opening at the end of the compression chamber while Tom cemented the various parts together. To make sure the valve stem was secure, sealed, and airtight, we used the spray foam to put a couple inches at the back of the chamber. A straw was used to make the air channel through the foam to the stem so it didn’t simply seal off the valve stem.

Once all of this was done, the PVC would take approximately 24 hours to cement together. So we left it overnight to cure. It was particularly difficult to resist trying it early. In the meantime, Tom and I made sabots for the golf balls to rest in out of spray foam. These are helpful because they sit tightly in the barrel, so air can’t escape around them. Within them the golf ball is cradled. When the spudgun is fired, the sabot and golf ball push out of the barrel, but due to the low density of the sabot, it just flutters off and lands a few feet away to be retrieved and used again.

The next day, we got our spudgun out and it was time to see how our handiwork functioned. First of all, it was surprisingly ergonomic for as large and home-made as it was.

Here I am holding the spudgun and standing next to Tom.

Preparing the spudgun to fire is fairly easy: Verify the valve is closed. Put the golf ball in the sabot. Ram the sabot down into the barrel using a broom handle or something similar until it stops. Then connect the air compressor to the valve stem and turn it on. Ours had an on-board pressure readout. Although ours seemed to give readouts much higher than in actuality while it was filling, probably due to the constrictive path the air takes through the straw, it would be careless to blindly fill up the tank without some sort of feedback as to what pressure you’re at. We could turn the compressor off and the readout would eventually stabilize down in the range we expected it to be.

A picture of our filling setup.

All in all, the spudgun we made up on Beaver Island was a great experience. We learned some new things while making it, and ended up with a really fun project. We don’t have perfect numbers as far as how it performed, but based on a couple videos I took of us shooting it vertically, I calculated the muzzle velocity of the golf ball at approximately 40m/s (131 feet/second). For reference, this is approximately 15% of the typical muzzle velocity of a .45ACP handgun round. Of course, our round is much larger and heavier, at about 3x the mass of the bullet. This all means the golf ball launched out of our spudgun has about 45% of the momentum of the bullet. Needless to say, this is not a projectile you would want to stand in front of.

While our project had many dangers to it, we exercised as much caution as we possibly could. Ahead of time, we researched the psi ratings of the PVC we would be using, so that we would not inadvertently put ourselves in danger of overpressurization. As it turned out, our lowest-rated component was the valve (I think it was 120psi), so our pressures were kept significantly lower than that maximum. I believe we typically ran around 50-75% of that maximum, based on estimates we would always pad toward the conservative side of things to further ensure our safety. Even in a worst-case scenario, we wanted to still have a margin of safety.

My only regret is that we had to leave the spudgun up on Beaver Island (I’m not sure what kind of looks we would have gotten taking it on the ferry, but that would have been hilarious). So, we’ll have to play with it more next time.

Videos

Tom shoots a 45-degree shot
Tom shoots straight up
Tony shoots straight up
Tom shoots water

All content ?2006 Tony Magri

Posted by nitrogen on Friday, November 17th, 2006 at 1:24 pm.