Symbolism, Imagery, Allegory
Russian author Anton Chekhov famously wrote that if a gun is shown in one scene, it has to be fired in another, otherwise it has no right to be in the story at all. This theory of composition is commonly known as "Chekhov's Gun."
We don't completely agree with a literal interpretation of the "Gun" theory. A gun can function, like here, as symbol of authority, a symbol of a constant threat of violence, whether it is literally fired or not. But we can extend this theory to mean that the images in a given scene are significant function towards an ultimate goals of the work as a whole. However you feel about "Chekhov's Gun" the gun in "Dry September" is a brilliant touch.
If we were to take the theory literally, and assume that Faulkner meant to employ it in this story, we would have another piece of evidence suggesting that Will was killed after Hawkshaw jumped from the car. The gun is first seen in Part 1, and Part 3 (featuring Will's abduction) would be the natural place for it to be fired. But, we can't assume that it was. The gun is seen for the second and last time in Part 5:
He [McLendon] took the pistol from his hip and laid it on the table beside the bed. (4.7)
There is nothing to let us know whether the gun was fired, or not (and Will certainly could have been killed without a fired gun), but it presents a symbol of past and future violence.