First, put the Smaller tin into the Altoids tin, then put in the C02. It should be a snug fit, but it may depend on if you use a tin other than the J...
Have you ever been pinned down in an airsoft battle and you are out of ammo or Co2? If so, then this is the quick and easy project for you. This project conceals a Co2 cartridge and about 275 pellets in one altoids tin.
-Jones Carbonated Candy Tin
-Ammo of your choice