One of my Airsoft guns is a HK USP CO2 pistol.
The way it is designed, the CO2 cartridge sits in the magazine. This is nice, because all you have to do to render the gun completely safe is remove the magazine.
The only problem is that it requires an allen wrench to take out the plug and change a cartridge. The wrench seems to get lost easily, so I did this so I wasn't stuck with just using the wrench.
Step 1: Tool
I used a Dremel tool with a cut-off wheel, but a saw or a determined person with a knife could do this easily.
Step 2: Find the Peice
Take the magazine out, make sure there isn't a cartridge in it (obv.), and remove the plug on the bottom.
Step 3: Cut It
Cut a notch across the wrench hole as shown. I wouldn't cut deeper than about an eighth of an inch. Clean up any plastic burrs around the cut when done.
Step 4: Screw It
Now you can use a coin, a knife, or pretty much anything that will fit in the notch and not break to change cartridges on your magazine.
Any Questions? I do hope I made everything clear...