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
The leaky gun in question
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.