This will show you how to make a cheap airsoft gun out of a cap gun. This is not to be relied on in an actual airsoft battle. It's just a fun project to try.
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7-31-09: Today I found that 12hippos posted the same project as me, but about 6 months earlier. Please note that I did not copy him, as I did not know the instructable existed. If you really want to see heres the link. https://www.instructables.com/id/SUPER_Cheap_Airsoft_Pistol/

Step 1: Get Your Supplies

You will need a cap gun revolver. I got this one at Albertsons for 2 bucks
You also need caps. I got a pack of 24 cap rings for a buck at Albertsons
You need airsoft bb's (of course). You can get these at wal-mart or any sporting goods store.

Step 2: Widen the Barrel

Use a knife or a pair of scissors to widen the barrel so a bb can pass through. Make sure it's wide enough by dropping a bb into it. The bb should pass right through.

Step 3: Insert the BB's

The gun I have has a removable cylinder, which makes it easier to put in the bb's. Check the picture to see where to insert the BB's. After you have put the bb's in, you need to push them down as far as they will go. I used a screwdriver.

Step 4: Reassemble the Gun

Put the cylinder back in (if you removed it) and put the cap ring in,

Step 5: Use Your New Airsoft Gun!

Your airsoft capgun is now ready to fire. Always use eye protection when firing the gun, the bb's are prone to ricochet.

Step 6: Improving Accuracy(optional)

I managed to increase the accuracy of the gun by hot gluing part of a pen tube to the barrel. Another section of the pen tube is placed inside the gun, as seen in the picture.