How to make a breech-loading barrel for a spudgun
Rather than build a typical interchangeable barrel with the threaded fitting on one end like this:
These are instructions to make a breech-loading barrel like this:
For use in a launcher like this:
First, select materials. In this example, I used:
1. Pressure rated PVC pipe for the barrel, 1.5” diameter, 10’ long
2. A 2“threaded male adapter
3. A 1.5” to 2” reducing bushing.
For the adapter and bushing, I prefer to use PW fittings rather than DWV fittings. The difference can be seen in the image below, with the weaker DWV on the left, and the stronger PW on the right. All of these fittings are schedule 40, but only the PW fittings are pressure rated.
Most of the work involves modifying the reducer in order to allow the barrel to pass through the fitting.
The first step is to simply remove the ridge on the inside of the reducing bushing. The location of this ridge is shown by the arrows below:
Some people use a dremel tool or even a lathe to remove the ridge. Not me. I use a good old-fashioned file with a curved face:
Using my file, I can remove the ridge within a minute or two.
After the ridge is removed, then real work begins. This type of fitting also has a slight taper that will need to be removed in order to allow the barrel to pass through the fitting. Try to work carefully on this step, filing a little and then checking to see if a barrel will pass through. It takes me about 20 minutes of careful filing complete this step:
Once the barrel passes through, you will need to remove a bit more material form the inside of the fitting because otherwise you will not be able to complete the assembly. This fitting should slide fairly easily over the barrel, and you should not have to twist it and fight it in order to get it in place. If you need channel lock pliers to move the fitting, then you need to file more.
Then when you can slide the reducing fitting into place, you should attempt a dry assembly in order to make sure everything fits and is pointing in the correct direction:
Go ahead and put everything together without using PVC primer or cement:
and make sure the barrel breech extends as far as you need:
For this particular barrel, I extended it past the end of the launcher to make it easier to load spuds without having to remove the barrel to reload. Instead of a typical cap, I had to create a chamber extension to accommodate the extended breech of this barrel. The chamber extension looks like this:
I have also built barrels that did not extend past the end of the chamber, but instead stopped short of the end:
and those barrels worked well, but were more difficult to load unless the barrel was removed from the launcher after each shot.
Now use a pencil to mark the front edge of the fittings on the barrel:
Then remove the launcher and threaded adapter so you can mark the rear of the reducer fitting:
When you slide the reducing fitting away, you should have something like this:
Now go ahead and assemble the fittings. First prime, then apply the PVC cement:
Here are the assembled fittings, almost ready to be installed on the barrel:
But first check to make sure they are still able to slide easily onto the barrel. I have noticed that the process of joining these fittings seems to change the shape of the inner fitting. So if it has changed, then you may need to file the inside surface a bit more.
When you are satisfied that the fitting is ready to install on the barrel, you can then prime and cement the area of the barrel between the pencil marks that you made earlier:
Be sure to apply a thick coat of PVC cement:
Then apply only primer (not cement) to the inside of the fitting and quickly slide it into place:
The arrows show the beginning and end of the path traveled by the fitting. When then fitting reaches the area of the barrel where you previously applied primer and cement, you’ll notice that a lot of the cement is scraped off by the fitting. Therefore, you need to spread the cement by working the fitting back and forth across the cemented area several times, and then give the fitting a good 1/4 turn twist.
Finally, use a file or other similar tool to add a cutting edge to the breech of the barrel:
When this is complete, you can load spuds in your potato gun without having the hassle of a ramrod. I really enjoy this feature on my spud launcher.
A few comments on how I managed to screw this up one time.
One time I did not remove enough of the inner taper from the 1.5” to 2” reducer. When it was dry, I could barely slide it over the barrel if I twisted and pushed really hard. I thought that was good, b/c I wanted a strong connection. But here’s what I ended up with:
and the fitting never reached it’s destination.
Although that was very frustrating, I was still able to use the barrel by simply repeating the procedure on the other end of the barrel. This gave the barrel a typical connection on one end and a breech connection on the other. I think it’ll actually be a great barrel for comparing breech versus typical loading and launching. So this little accident has inspired me to do some chrony testing. Stay tuned for results.