This one goes into the "completely impractical but entertaining" folder.

Turning a cheap cap gun into a single-shot Airsoft pistol is simple, cheap, and guarantees hours of fun, welts, and ringing ears for all.

Okay, so not all of those things are entirely good, but I will shut up and tell you how to make this already.

Step 1: Parts

These only cost about 5$ a piece, and with the materials you buy, you can make many guns.

You need:

Cheap cap-gun. The only requirement is that it uses rotary caps, the kind that come in a little ring. That even is debatable. You might be able to make other kinds of guns work. I had two different designs of gun, but once you cut the front off, they are virtually indistinguishable.
A few inches of Airsoft barrel- I use 6mm aluminum tubing from my local Ace Hardware.
Brass tubing that the Airsoft Barrel fits in- I don't have an exact measurement, but you can get it in the same place you get the 6mm tubing. It is usually pretty cheap.
Piece of wood, about an inch square and five or so inches long.- Self explanatory, I hope
Hot glue. If you don't have any, you should.
Caps for the cap gun. I personally find that the rotary caps in the little blue holders work the best, but you can experiment to find the most effective ones.
A pen. a standard Bic pen should work fine, as long as a cap will fit over the end of the cone part. (this will be explained later)
-Small screw. (the noun, not the verb)

Step 2: Tools

This project should only require some basic tools.

Hacksaw or other multipurpose cutting thing. I used a drill-mounted abrasive wheel for all of the cutting in this project, but whatever works.

Hot glue gun- for hot gluing.

Anything that works to ream out the ends of cut tubing. I used a little grinder bit in my drill, but a screwdriver or even a junky pocket knife would work because aluminum and brass are soft metals.

A screwdriver

A sharp knife. I used the 15 dollar knife I keep on my work table for standard use and abuse.

Step 3: Hacking Up the Gun

Following the pictures, cut off the forward barrel and cylinder receiver. Smooth off any burrs or bumps.

Step 4: Cut the Wood

cut your piece of wood to length. round down the edges somewhat.

Notice when you pull the trigger, that a metal spur pushes out of the body of the gun. there may also be a plastic lever below that that similarly pokes out. We want the piece of wood totally flush to the body of the gun, so you must carve notches into one end of the wood to give clearance for these moving parts.

Refer to the pictures for help.

Step 5: Glue the Wood to the Gun

hot glue the wood to the gun as shown. make sure everything is flush and that the moving parts have plenty of clearance. Also, make sure that glue isn't anywhere it isn't supposed to be.

Step 6: Cut Tubing

Using your cutting gizmo, cut about an inch or so of brass tubing, and about four or five inches of aluminum Airsoft barrel. The lengths don't have to be exact, but the longer your brass tubing, the more support your barrel will have, and the longer the barrel, the more accurate your gun will be, to a certain extent.

Ream out the ends of the cut tubing, so the aluminum tubing slides cleanly inside the brass tubing.

Step 7: Glue Brass Tubing in Place

Glue the brass tubing in place, using the pictures for a reference. just use a little line of glue at first to get the position right, then put in on heavy, going over the tubing as well as around it to make sure it is on good and tight. Once the tube is attached, make sure the aluminum tubing can still slide freely in the brass tube, and that no glue got into the brass tube.

Step 8: Attack the Pen

Take the pen completely apart. Take the cone from the tip, and the pen tube, and throw the rest away.

Take the cone of the pen, and see if it fits in one end of the aluminum tubing. If it does, stick it in, making sure it is a tight fit and won't fall out. Also, make sure that the transition between the tubing and the cone is smooth, with no parts in the way.

If the cone does not fit, use a file, a rotary tool, a pocket knife, or anything that works to make it fit. Once the cone fits, jam it in.

The tube with the cone on it should slide through the brass tubing easily, and the tip of the cone should contact the tip of the gun's hammer perfectly.

When the cone is in place, test-fit a cap on the end. It should fit well, without falling off. You may have to shave down the tip of the cone to ensure a tight fit.

Step 9: Cut Barrel Clip and Install Screw

Cut a half-inch section of pen tube, then remove a 1/4 inch section from the side of it, so it can easily snap around the aluminum barrel.

Using the picture as a guide, put a screw right next to where the barrel goes, about 1/2 inch from the end of the gun.

Step 10: Finished

Your gun is finished! put it all together as shown in the picture.

Now let's go shoot it!

Step 11: Loading

Get a cap, an Airsoft BB, and a long thin thing for a ramrod.

I used the caps that come in a ring. I just cut them out individually with scissors.

Don't use expensive BBs for this. Use the cheapest ones you can find, as accuracy is going to be negligible beyond a few feet anyway.

I use the ink tube from the pen as a ramrod.

put the cap onto the back of the pen cone as shown. make sure it stays on. you might have to modify the tip to get it to stay.

put the BB in the barrel and push it all the way down. If it rolls out, don't worry, it will stay in after a few shots, from the accumulated crud from firing.

slide the barrel assembly into the gun as shown, all the way back, pushing the hammer back a tiny bit to ensure contact. Put the barrel clip (pen tube piece) on, braced up against the screw to hold it in place.

The gun is now loaded, so treat it accordingly.

Step 12: Shoot!

Put on ear and eye protection, and go outside. This isn't really an inside weapon.

Point at a tough inanimate target, pull the trigger, and enjoy!

This gun delivers a great bang, muzzle flash, and puff of smoke. The BB flies out extremely fast, but not always consistently, with enough power to occasionally over-penetrate a cereal box!

Step 13: Legalities, Warnings, and Such.

Some things about this gun that you should know:

-This is a firearm. That makes it illegal for pretty much anyone to own or use without proper permits, legal stuff, writs signed by the king himself, etc. I'm in the US. I'm not sure of foreign laws.

-Yes, there is no orange tip on this. This is for a reason. As stated above, this is a firearm. It is illegal to put an orange safety tip on a firearm. If you get caught with this and the people who catch you decide to punish you, you will get in more trouble if this has a safety tip.

-Since this is a firearm, follow all gun safety rules.

-Despite the above warnings, I say DO TRY THIS AT HOME, as long as it is in a safe environment. This includes safety goggles, and to a lesser extent, hearing protection.

-This is intended for target shooting, and not for actual Airsoft games. NEVER TAKE THIS TO AN OFFICIAL AIRSOFT FIELD. THEY WILL RUIN YOUR DAY IF YOU DO. Although this gun could easily be used in an Airsoft game, it would be practically useless. I am not going to say never do it, just don't use it in official matches, in public games, and tell the people you are playing with about it.

-If this gun is so questionable, then why did i post it? Because it is fun, that's why. Also, there are many things on this site that are WAAAYYY more illegal and dangerous than this. That's why.

Step 14: Troubleshooting

If you have trouble shooting, you need troubleshooting...

Ack.

Sorry.

Okay, here are some helpful hints to keep this thing working.

The cap doesn't explode- Make sure your cap is live, sometimes there are duds. Also, make sure the hammer fully contacts the cap when it snaps forward. make sure the barrel is fully in place and clipped.

The cap explodes, but the ammo does not shoot- The barrel may be dirty. get a q-tip and some rubbing alcohol or other semi-strong cleaner and swab the barrel out. Use a bit of wire to poke the ammo out. Try again with another cap, the one you just used might have been improperly placed or defective.

The BB goes in some random direction- This gun is usually really inaccurate as it is, but it usually shoots mostly straight. Make sure your barrel is clean (see above). make sure there are no metal burrs on the end of the barrel. If so, scrape it smooth with something.

The pen tip cone has exploded/cracked- Don't bother repairing it, just tear it out, and go get another pen tip off on another pen. They aren't too hard to find.

The trigger/hammer/gun in general does not work- Keep in mind, these guns are made from extremely cheap cap guns, and are prone to breakage in their normal function, not to mention modified use. Try to fix it if you can, but usually the best solution is to just snap off the old broken gun and glue another one onto the wooden block to replace it.

Any more questions? Gun fixes I didn't think of? Comment and I will try to help.

Step 15: Dueling Solves All Problems

Okay, so now is where I go against most of my other safety advice, and the real fun begins.

This is a duelling pistol, essentially, so why not duel with it?

Dueling is a fight to the death, used to defend honor, settle fights, and make dramatic Hollywood movies.
The major problem with duelling is that the loser dies, usually. Using this duelling pistol, you can get all of the satisfaction of defeating an enemy, without the messy problems that come with killing them.

The possibilities are endless. Deciding who gets the last beer at a party... Settling arguments over who does the dishes.... winning the heart of your bonnie lass. I say again, the possibilities are endless.

Here are the basic rules, adapted from Wikipedia, with safety steps added

1. Someone must challenge another. This can be in the form of throwing a glove at someone, or as simple as saying "I challenge you to a duel."

2. Location, and time are agreed upon. Stakes (what they are fighting about and what the winner gets) are agreed upon. Also agreed upon is whether or not seconds will be used. A second is someone who fights in your stead if you are killed. There must be at least one other person present to ensure fairness and call the start.

3. At the location and time, the duel is set up. A set number of paces is decided upon. The opponents are both given equal weapons, and placed back to back. The opponents both wear safety glasses, and preferably something to cover their face. Paintball masks are preferred.

4. Someone counts down and calls start. The opponents walk the set number of steps away from each other. Once they reach the set number of steps, they turn and fire. Running is prohibited.

5- If a hit is scored, that person loses, and must cede whatever the stakes were. If no hits are scored, then the challenger can chose whether or not to have another round. A match cannot be more than 3 rounds, even if no hits are scored. If seconds are used, they can continue to fight, but are usually not needed.

This is a fun and less-than lethal way to solve conflicts, or just to mess around. This is also a great spectator game.

Have fun. If you do duel, i would appreciate videos!