Build a gas-powered marshmallow shooter

This past December marked the 75th anniversary of Pearl Harbor, when nearly 400 Japanese aircraft attacked the American naval base. In the space of a few hours, the U.S. Pacific fleet was nearly destroyed. And after the assault, America invested heavily in improved anti-aircraft weapons.

Designing anti-aircraft, or ack-ack, guns was difficult. Because airplanes move so fast and have such extreme maneuverability, gunners must shoot at the target’s future position, making the aiming mechanisms quite complex. So America’s new guns were complicated and expensive—but boy, were they effective. The fast-firing autocannons could paint the sky with lead, firing more than 120 anti-aircraft shells per second. On one occasion in 1942, the guns aboard the U.S. battleship South Dakota shot down 32 enemy planes in a half hour.

I designed a fast-firing weapon that conjures up ghosts of the Bofors and Oerlikon anti-aircraft guns of World War II. Since it’s for home use, I powered it with gas instead of gunpowder and made it shoot mini-marshmallows instead of two-pound lead slugs. And of course, I call this marshmallow ack-ack a “mack-mack” gun.

Made from PVC pipe and fittings, a bicycle tire inflator, and easily obtainable cartridges of compressed carbon dioxide, the mack-mack is cheap, easy to make, and can be assembled in minutes. You may have seen marshmallow guns that use lung-power to shoot single mini-marshmallows, but this high-velocity weapon is far more impressive.

After assembling the PVC pipe and fittings, just as I would for a standard marshmallow gun, I drilled and tapped a single hole in the plastic and attached the bicycle inflator. The inflator releases a burst of compressed gas that’s much more powerful than a puff of breath. I also gave the gun another modification: an ammo tube that can hold seven mini-marshmallows at once.

When I pull the inflator’s trigger, all seven marshmallows fire in a single half-second, shooting out one after another in a glorious fusillade of soft, sugary firepower. Want to build your own mack-mack gun? Check out the instructions below.

The author fires his gas-powered marshmallow shooter Photograph by Ackerman + Gruber

Build your own mack-mack

Tools

Tape measure
Electric drill
7/16-inch bit
¼-inch pipe tap and handle *
Adjustable wrench

* A ¼-inch pipe tap is different than a ¼-inch UNC tap

Materials

All parts are ½-inch diameter, schedule 40 PVC pipe or pipe fittings unless noted otherwise.

16-inch-long PVC pipe
Three 4-inch long PVC pipes
Two 2-inch long PVC pipes
PVC coupling fitting
Two PVC tee fittings
Two PVC 45-degree elbow fittings
Three PVC pipe caps
CO₂-powered bicycle tire inflator with several spare 16 gram CO₂ cartridges *
Air tank valve, ¼ NPT to Schrader ^†
Mini-marshmallows

* Try finding these at a bike store or search online for “CO₂ bike pump” or “CO₂ bike tire inflator.”

^† These valves are available at hardware and home stores with reasonably large inventories. You can also find them online by Internet searching for “air tank valve” or “tru-flate valve.” Another alternative is to order from a large supply industrial company like McMaster-Carr.

Instructions

Make a threaded hole in one of the pipe caps so you can attach the tire inflator: First, drill a hole in the center of one of the pipe caps. Now you’ll need to “tap” the hole, or cut screw threads into it. The plastic cuts easily, so you shouldn’t have much trouble with this.
Once the hole is tapped, insert the tank valve into the hole and tighten using the adjustable wrench.
Assemble the mack-mack as shown in the assembly diagram above. Press fit the pipe pieces into the sockets of the fittings, but do not use PVC cement or primer.
Screw the CO₂ bike tire inflator into the Schrader fitting side of the air tank valve.
Now the marshmallow shooter is ready to go! In the world of automatic weaponry, burst fire mode means you can fire a predetermined number of projectiles with a single pull of the trigger. Here’s how to use it.
Remove the cap from the upward-pointing 4-inch pipe.
Insert six or seven mini-marshmallows into the uncapped pipe, making sure they can fall freely into the main gun barrel. If the marshmallows are gooey and stick together, coat them with flour. Replace the cap.
Put on safety glasses and choose a target wisely. To shoot, pull the trigger on the bicycle tire inflator and release quickly. Go easy on the trigger—holding it for too long will quickly deplete your CO₂ supply and cause the cartridge to ice over. If that happens, you’ll need to wait until it thaws before you can fire again.
The mini-marshmallows will shoot out one after another. Expect to get about six to eight bursts per gas cartridge.

This project is from Ready the Cannons, a new book by Popular Science contributing editor William Gurstelle.