In Part II of The Stranger, Camus uses a mix of visual and auditory imagery that is connected violently to nature and the body in order to show the gun shoots itself instead of Meursault consciously pulling the trigger.
Meursault narrates (imagery in bold):
At the same instant the sweat in my eyebrows dripped down over my eyelids all at once and covered them with a warm, thick film. My eyes were blinded behind the curtain of tears and salt. All I could feel were the cymbals of sunlight crashing on my forehead and, indistinctly, the dazzling spear flying up from the knife in front of me. The scorching blade slashed at my eyelashes and stabbed at my stinging eyes. That’s when everything began to reel. The sea carried up a thick, fiery breath. It seemed to me as if the sky split open from one end to the other to rain down fire. My whole being tensed and I squeezed my hand around the revolver. The trigger gave; I felt the smooth underside of the butt; and there, in that noise, sharp and deafening at the same time, is where it all started. I shook off the sweat and sun. I knew that I had shattered the harmony of the day, the exceptional silence of a beach where I’d been happy. Then I fired four more times at the motionless body where the bullets lodged without leaving a trace. And it was like knocking four quick times on the door of unhappiness.
The scene contrasts heavily with the earlier beach scene in Part I in which Meursault and Marie frolicked and flirted. Here, the natural elements (sun, sea) have almost conspired against Meursault, stinging and covering his eyes, blinding him. This is the imagery of an absurd universe: one day it is inviting and free and the next day it is violent and cruel.
Earlier, the sun, sea, and sand were symbols of freedom, but hear they are connected to violence and unhappiness. The unbearable heat for Meursault in Part II here is much like the unbearable heat for Perez during the funeral procession in Part I. Both men were victims of determinism; they lacked freedom because they attached themselves to death. Meursault gets involved in Raymond's revenge plan and takes his gun; Perez forces himself to mourn another's death out of an over-developed sense of grieving. In both cases, the universe punishes them dearly, nearly killing Perez and leading to Meursault's execution.