From above forum, and it is valid for --ANY-- garbage collected language:
(Note: GC deletion from memory is not immediate.)
In your code, make sure that your bullet is no longer visible and becomes 'disabled' once it hits a target. You cannot expect the GC to remove it in the very moment you call delete(bullet); GCs work asynchronously from your own code, and delete() only marks an object for deletion, it does not actually delete it.
(Note: GC deletion from memory is not immediate.)
In your code, make sure that your bullet is no longer visible and becomes 'disabled' once it hits a target. You cannot expect the GC to remove it in the very moment you call delete(bullet); GCs work asynchronously from your own code, and delete() only marks an object for deletion, it does not actually delete it.