In my instructable "Monkey Hunter Physics!", which involves an advanced build, I said I'd include pics of the simpler gun build for non metal workers. Here it is.
Simpler Gun for Non Metal Workers:
Projectile: 9/16" ball bearing.
Barrel: 1/2" copper tubing, _Type M_. DON"T USE Type L, it has a thicker wall and won't fit the ball bearing.
Hardware stores will cut pipe to length. 2 to 3 feet will work. Tubing cutters leave a burr inside the pipe. Deburr the top end. Having a burr at the bottom is actually good, since it will keep the ball from falling out the bottom end.
Mouthpiece: 5/8" ID clear flexible tubing. Since this is flexible, you won't mess up the aim. It's a tight fit over the bottom end of the copper pipe.
Barrel Supports: 2 Test tube stands borrowed from the chemistry department. http://www.indigo.com/science-supplies/gph-science-supply/clamp-ring-standware.jpg The type that clamps down on a test tube and is adjustable for height and angle.
Just as with the large gun, the connections for the wire at the end of the barrel need to be insulated from the barrel. 2 insulated alligator clips do this nicely.
Aim directly at the target.
Blow soft, or blow hard, it should hit the target as long as the target doesn't hit the ground first.
Here's the link to the video: http://www.youtube.com/watch?v=N9lvazPYFUE
...and instructions for the other parts:
Making the electromagnet
1) Find a bolt, 2 nuts and 2 fender washers. Cover the washers and bolt with plastic tape. This will prevent the coils shorting out if cracks develop in the magnet wire's varinsh insulation. Assemble bolt, nuts and washers to make something that looks like a spool.
2) Mount your supply of magnet wire on an axle so it can freely turn. Secure the bolt assembly in a drill. I used a cordless. Wind the wire onto your spool assembly. A drill press or lathe could also be used. If you don't have any of these, a coil can be turned by hand, it will just take time.
3) Tape over the coil to keep it from unwinding. 2 leads should be accessible. scrape/sandpaper/burn the insulation from these leads. Solder some heavier wire to these leads, and secure the heavy wire to the coil with tape. This will prevent strain from being put directly on the delicate magnet wire.
4) Test to see if the electromagnet is strong enough to hold up the can.
The "Monkey" Target
1) Cut both ends from a tin can.
2) Stretch panty hose over 1 end to catch the ball. Tissue paper over the other end will give a surface that a face or target can be drawn on.
3) My magnet was not strong enough to hold the can up until I drilled a hole in the can and put a bolt through this. A piece of plumbers tape bent into a bracket and secured to this bolt keeps the can from rotating while suspended from the magnet.
1) The electromagnet is suspended fairly high. I used extension cord to connect it. It is heavy gauge, and long enough for the purpose. Do not apply house current to the magnet. An 18 V battery is enough, and 110 V would likey burn up the magnet.
2) Another piece of magnet wire will be stretched across the muzzle of the gun. (use tape to insulate this FROM the barrel) When the short piece of wire is broken by the ball being launched, the electromagnet will drop the can. Therefore, magnet, battery and the short wire are connected in series.
3)Sight along the tube to align with the target. Blow to launch.