I would highly recommend that you do not make your own airsoft pellets. They might jam and or brake your gun.
The BBs made in the factory are almost flawless, making a homemade BB and using it would be very bad for your gun. One of the easiest ways to break your gun is by using bad BBs. Also do not reuse a fired BB or use .12g BBs in an airsoft gun unless it is a spring rifle or pistol. Make sure you use .12g BBs or higher. BBs are extremely cheap as they are only around $15.
it depends on the gun. read the box and it should have a recommended size ammo for the gun. machine guns probably .20s at the least most pistols work with .12s but are better with .20s snipers you want .25 or higher spring rifles and shotguns anything will work but i like to use .20 or .25
Airsoft weapons do not fire real ammunition.
In the magazine.
The bullets used in an Airsoft gun are BB's but an owner should make sure that pay attention.to the quality of BB's that they buy. If they are sure to buy the best BB's the gearbox components will be damaged and cause the owner to spend far more cash over the long haul.