What do the guns symbolize in "The Sniper" and what is a quote that shows their symbolism?

The guns are (literally) weapons that cause injury, death, and destruction. Symbolically, they represent violence. In the first paragraph, the narrator says the guns "roared" and follows with this description:

Here and there through the city, machine guns and rifles broke the silence of the night, spasmodically, like dogs barking on lone farms.

The guns "roared" and "barked" as if they were living things. Here, the narrator uses a kind of zoomorphism. This is when people or objects are given qualities usually reserved for animals. The guns are not whimpering and whining. Rather, they are illustrated with these more aggressive animal calls. This shows the predatory nature of the guns and the potential violence characteristic of a predator.
Near the end of the story, after the main character has killed the enemy sniper, he becomes sickened by his violent act. He throws the revolver down in anger. Once again, the revolver acts as if is a predatory animal with a life of its own:

He looked at the smoking revolver in his hand, and with an oath he hurled it to the roof at his feet. The revolver went off with a concussion and the bullet whizzed past the sniper’s head.