As we mentioned above, old BB pellets are likely to be dirty. When they are inserted into the inner barrel of your gun the dirt can collect on the sides and build up. This massively compromises the gun’s airflow and means that you will have to clean the gun much more frequently.
If the BB pellet you are reusing is damaged in any way, this can cause further damage to your inner barrel. If there are sharp exposed edges on the pellet this can scratch the interior. When this happens consistently the shape of the inner barrel is irrevocably altered and the gun’s accuracy is greatly reduced. If you notice the damage early you may be able to fix it through polishing, but the performance will never be the same as before.