Built like a small potato cannon, this mini-marshmallow-firing musket is not a thing of engineering beauty but its ultra-low cost and easy assembly make it good for mass production.

Step 1: Gather Tools and Materials

tools (available at any hardware store):

PVC cement
PVC primer
a PVC pipe cutter, hacksaw, or some other way to cut through PVC pipe
a power drill
a 3/32" drill bit (just a tiny bit larger than the needle of a ball pump)
a file or knife to clean the ends of the PVC pipes

materials from the hardware store:

about 3' of 1/2" schedule 40 PVC
about 2' of 1" schedule 40 PVC
1 1" PVC ball valve
1 1" PVC cap
1 1" --> 1/2" threaded PVC reducer bushing
1 1/2" --> 1/2" threaded adapter

materials from the dollar store:

plumber's tape
1 ball pump
4 zip ties

Step 2: Cut the PVC Pipe to Length

Lengths don't have to be precise for this, so just eyeball it: I use about 2' of the 1" PVC for the pressure chamber and about 3' of 1/2" for the barrel.

Use your saw or PVC pipe cutter to cut to the appropriate length, then use a file or blade to clean up the area of the cut so it will fit in to the fittings.

Step 3: Dry Fitting

Fit all the parts together to make sure you understand the layout and that everything does in fact fit.

From front to back the order of the parts will be:

barrel of 1/2" pipe (will be glued to the...)
1/2" --> 1/2" threaded adapter (will screw into the...)
1" --> 1/2" threaded bushing (will be glued to the...)
ball valve (will be glued to the...)
compression chamber of 1" pipe (will be glued to the...)
end cap

Step 4: Glue!

Now cement (almost) everything together. Do NOT cement the two threaded bits to each other.

To glue PVC, clean the area to be cemented then swab both parts with a dab of purple PVC primer. This sets almost instantly, allowing you to apply the cement. PVC cement is nasty stuff - use ventilation and don't get any on your skin. Like hair gel, a dab'll do ya. Once the cement is applied, fit the parts together tightly. Use a paper towel to wipe up any excess. Allow five minutes to dry.

Step 5: Drill

Air will be pumped into the compression chamber through the needle of the ball pump. Pick a point about half way down the end cap and use a 3/32" bit to drill a hole diagonally through cap. The location is to keep the outside part of the needle from bending when the musket is rested on end. The angle is to give the needle the maximum possible material to go through, for stability.

Plan your angle carefully, though - you don't want the needle bending when you rest the musket, but you also don't want the needle to miss the pipe.

Step 6: Prepare the Needle

Because the hole is slightly larger than the needle (believe me, I tried making the hole slightly smaller and the needle just bent when inserted), you will need to wrap the needle in a bit of plumber's tape. 2-3" should do. Start at a relatively steep angle, maybe 45 degrees, but as you wrap it around gradually reduce the angle so that by the end you're wrapping it straight around. Think of it as building up a very slight cone around the needle, tapering toward the tip, so that it can find its own fit in the hole.

Step 7: Attach the Pump

Stick the needle in the hole. There will be some tape bunching up, but it should hold pressure (for a while - this is not a perfect design, merely a cheap and easy one).

Attach the pump itself to the compression chamber using the zip ties. You will probably need to attach two ties to each other to get all the way around. Once it is on tight, clip the excess and you're done.

Step 8: Operation

This musket is designed to fire mini-marshmallows, but should be able to fire bits of potato, apple, grubs, or anything else you can fit in the tube.

If you are using something that won't easily slide down the tube, the threaded connection between the barrel and the ball valve can be undone and ammo placed in the barrel from that end.

If muzzle loading, be sure the valve is in the close position when dropping in ammo.

Close the valve and compress the pump about five times. Aim and release the valve to fire.