If you have any experience with Airsoft, you probably know that CO2 pistols like to leak.

There are two types of leaks: Valve leaks and seal leaks.

If gas is coming out the barrel, that is a valve leak and a lot harder to fix.

If gas is coming out the seal around the tip of the cartridge, that is a seal leak.
This is how to fix seal leaks.

Step 1: Materials

You need:

The leaky gun in question
Full cartridge
Teflon plumbers tape, available at any hardware store that is worth shopping at.
Silicone spray lubricant or Paintball marker oil. Do not use any other type of oil, it will freeze and damage the seal even more.

Step 2: Oil the Seal

Find the seal. If your gun has the CO2 holder in the handle, look up inside that. If your gun loads CO2 into the magazine, look up in there. My gun has the gas in the mag.

Anyway, put a good amount of lubricant on the seal. you should do this after every game before you put the gun away, and, if at all possible, each time you load a new cartridge. Basically, the more often the seal is lubricated, the better.

Step 3: Tape the Cartridge

Get your new cartridge and wrap about three wraps of Teflon around it. Teflon tape is annoying to work with, as it sticks to itself and is gossamer-thin. It might take a try or two to get good at it. Don't worry if you waste a bit of tape, it's cheap and you don't use much anyway. Once the tape is wrapped, smooth it down with a finger tip.

Then, put some more lubricant on the taped head.

Refer to the pictures.

Yes, you can prepare multiple of these before you play a game, just make sure that the tape isn't unwrapped.

Step 4: Load and Test

Load the cartridge like normal. It should leak a lot less. This is not guaranteed to work, but it can't hurt.

Remember, lubricating your seals is a good habit to get into!