I know this has been suggested before, but I think I may have thought of a way to make it not too op but still a fun weapon to use.
Well, I have 2 ideas that is.
Have the pigeon be controlled by the user with dodgy controls, if they land and hit the target then the pigeon immediately kills them.
Have the pigeon home on to the nearest target, have it be very slow and only do 75 - 85 damage on direct hit so you have to finish off the target with your gun. If you hit the pigeon while it's mid air it falls on the ground and doesn't do anything.