When selecting ammo for BB guns remember that quality counts. Budget models which are under 0.3 joules in power (usually Pistols under £20.00, Rifles under £50.00) will usually work well with cheaper ammo, using lightweight 0.11g or 0.12g pellets. Good quality lightweight pellets will also work well in more powerful models but mediumweight 0.2g or heavyweight 0.25g will give better range and accuracy. Experiment to find the best ammo for your gun.
Re-using BB's can be a big mistake as if they have become distorted they'll jam the gun, particularly more expensive models which are made to finer tolerances. The same can be true if inferior, very cheap BB's are used. If they prove to be misshapen then jams can occur. Most problems with quality BB guns are caused by poor ammo.