The 20mm “Vulcan” Cannon
“The 20x102mm (or “20mm “vulcan”) cannon was an idea I got after seeing a lightly-used barrel for sale on Gunbroker for $900. I bought the barrel in about 2013, submitted an ATF Form 1 to build a Destructive Device… and then did nothing for about 2 years.”
-Josh
The 20mm Cannon Project
The 20mm cannon is an icon among kommandos around the United States. In 2016 Josh Geho had finally gained motivation to start the project, and so he began blueprints for the mechanical action. He knew it needed a simple action that was robust, and (most importantly) easy for an amateur to machine. After some thought he decided to go with a “falling block” action, except the block would be raised manually by hand. The user would pull on a chain attached to the breech block in order to do this.
To manufacture the gun, Josh bought a 3 x 36″ bar of 4140 steel off of eBay for a price he considered fair. In his words, “4140 steel is a pretty good garden-variety grade of gun steel”. It is somewhat easy to machine, has exceptional toughness, and is able to take some degree of hardening. All of this while being relatively inexpensive. From this 36″ long bar, he made almost every piece of the gun by slicing off bits at a time with his bandsaw.
Unlike all other traditional small arms, the cartridges for the 20x102mm use electric rifle primers natively.
The firing pin (or more accurately named, “electrode”) is a piece of 303 stainless steel that is completely insulated from the breech block with acetal (AKA “delrin”). From reading military manuals and case studies, Josh got a good feel for the voltage, current, and even the time required for reliable ignition. He constructed his first detonator on the inside of a Nintendo 64 controller for nostalgia and humor’s sake.
How is it powered?
The circuit is powered by two CR123 batteries. You press and hold one button to feed the current from the batteries into a transformer that steps it up to 240V, and then stores it in a flash photo capacitor. Once charged, you press another button to dump the stored current into the primer. It’s a violent charge, and will blast the finish off metal if you let it make a spark. It’s probably overkill to ignite a primer, but it works. “I wouldn’t want to get zapped by it”. This first detonator eventually got destroyed when it was left in light rain, and some switches shorted out and melted as a result.
Josh has since created a second detonator which is much more robust, although he calls it “plain and uninspired”. He made a mount for the cannon to sit on an M2 tripod. Not to be confused with the M3 tripod, for the M2 machine gun. He borrowed this piece from his 1919. The cannon is steered with a traverse & elevation adapter (T/E). For a sight, he used a “See All Open Sight, gen. 1”, which works surprisingly well. Josh has put over 30 rounds through the cannon. This powerful ammo was responsible for bending and breaking the legs of the tripod from the massive recoil.
The Ammo Situation
As of right now, ammo is difficult to find for this behemoth. You can’t just waltz into Cabellas and ask for a box of “electric primed 20mm Vulcan ammo”. Most of the ammo Josh has found so far is from the connections he has made with vendors at gun shows and saved searches on Gunbroker. He typically shoots M55A2 TP (target practice) cartridges out of his cannon. The M55A2 fires a ~1530 grain bullet at nearly 3300FPS. On average, Josh has paid around $11 per round, delivered. He sells the empty cases for around $2 a piece afterwards, which helps offset some of the cost.
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