This is a way to build an awesome magnetic rifle for under 30 dollars. Once you get all of the materials, it shouldn't take more than a half-hour to build.
What is a gauss rifle?
A gauss rifle is a device that takes advantage of a magnetic chain reaction to accelerate a projectile to high speeds. As explained below, the concepts of magnetism and conservation of momentum allow this really cool technology to work.
There is a lot of confusion surrounding gauss rifles; the word is often seen as interchangeable with "coilgun" or "railgun," which are in fact two separate technologies. Much of this confusion is caused by videogames such as Mechwarrior or Quake. A good rule of thumb is: anything you hear about gauss rifles, coilguns or railguns in a video game is probably wrong.
A gauss rifle uses a series of fixed magnets that work in sequence to accelerate a ferromagnetic (i.e. sticks to a magnet) projectile. On the other hand, a railgun uses a current-carrying substance placed in a constant magnetic field, while a coilgun accelerates a projectile in the center of a coil of wire, or solenoid. Both railguns and coilguns are much more complicated than gauss rifles, which I will be focusing on here.
Materials
You will need 4 neodymium gold-plated permanent magnets (12mm), 9 steel ball bearings of 1/2 inch diameter or similar, a wooden ruler with groove in the center, and strong tape.
Neodymium magnets and ball bearings are tricky to find if you don't know where to look; your best bet is probably on science hobby sites. I use www.scitoys.com, just search for "gauss rifle" or "magnets."
WARNING: Neodymium magnets are incredibly powerful. Keep the four magnets well separated from each other prior to securing them firmly to a fixed surface. If allowed to crash into each other, these magnets will probably shatter.
Construction
1. Using a marker, put four equally spaced marks on the ruler. If this is done correctly, you should have divided the ruler into 5 equal sections.
2. Place one of your magnets on the first mark you made. Make sure that the sides of the magnet are parallel to the sides of the ruler. Then tape the magnet VERY securely to the ruler. Don't be shy, use 3 or 4 turns of tape. That magnet is going to be taking a serious beating very soon, so make sure it's not going anywhere. NOTE: Don't use glue to affix the magnets to the ruler; this keeps you from being able to play around with the design later.
3. Repeat Step 2 three times for the remaining magnets, placing each on the next mark.
4. Pick a front end and a back end of your ruler. This is arbitrary; it just makes the next few steps easier to explain. You will start the reaction from the end, and the final projectile will come out the front.
5. Place two ball bearings in the groove in front of your front-end magnet. They should line up and stick to the magnet.
6. Repeat step 5 for the remaining 3 magnets. You should have 1 ball bearing left. Your construction should look like this:
I______#OO_____#OO_____#OO_____#OO_____
(where ____ is an empty section of track, # is a magnet and O is a ball bearing. Please excuse the awful diagram.)
7. Place your construction on a flat, level surface and point the front end away from people, lamps, small animals, large animals, delicate objects, and things you do not want broken. This rifle shoots the ball bearing at a very high speed, and if you're not careful you may get a nasty surprise.
8. Place the last ball bearing in the groove on the end of the ruler. It should roll towards and strike the end magnet, starting the chain reaction that fires the rifle.
To ready the rifle for its next firing, replace the 8 internal ball bearings in their original positions. Repeat step 8 to fire again.
Explanation
When the last ball bearing is released, it is attracted to the end magnet. It therefore accelerates towards and then strikes the end magnet.
IO-->___#OO____#OO____#OO____#OO____
The momentum is then transferred from the ball bearing through the magnet to the ball bearing on the other side. This momentum causes the new ball bearing to break away from the magnet and move towards the front.
I______O#OO-->__#OO____#OO____#OO____
However, there is another magnet in its way. It accerates towards this magnet, then hits it, repeating the process.
I______O#O____O#OO-->__#OO____#OO____
This continues for however many magnet/ballbearing stages you stick into the gauss rifle. Once you get to the last ball bearing, however, it simply leaves the rifle at a high speed after recieving the momentum from all the other stages. This, in effect, "fires" the last ball bearing out of the rifle.
I______O#O____O#O__-->O#O__-->O#O____O-->
Using these magnets and messing around with different configurations, I have managed to put a small hole in the concrete wall of my basement. Try different setups and see which works best.
What I have described here is a very simple kind of gauss rifle that isn't terribly efficient. You lose a lot of energy because each ball bearing, as it is speeding up, is slowed down somewhat because there are magnets behind it pulling it back. This problem can be solved, but requires a much more involved and complicated design. A setup involving pulsing electromagnets seems to be the most promising way of addressing this issue.
Gauss rifles are still troublesome enough that many military research programs have abandoned related projects due to inefficiency or severe battlefield limitations. However, it is likely that the problems involved with producing an effective gauss rifle will be solved in time. In such a case, make sure you are the first person on your block that gets one.