Answer:
Whereas the mockingbird is a symbol of innocence and all that is pious, the gun is (most likely) racism, deciet, and discrimination. The reason that Atticus didn't let Scout and Jem shoot a mockingbird was because it sang and made beautiful music; it didn't hurt or kill animals or other.
You could stretch it farther and say that Tom Robinson was the mockingbird and Bob Ewell is the gun.
Hope this helps!