In The Catcher in the Rye, Holden first acted out his "bullet in the guts" routine in chapter 14, after his altercation with Maurice:
About halfway to the bathroom, I sort of started pretending I had a bullet in my guts. Old Maurice had plugged me. Now I was on the way to the bathroom to get a good shot of bourbon or something to steady my nerves and help me really go into action.
Holden starts play-acting to avoid reality. He imagines himself as a character in a noir thriller or a pulp fiction magazine because this gives a superficial glamor to his squalid situation. In chapter 20, when he is drunk, he reverts to this fantasy again:
When I was really drunk, I started that stupid business with the bullet in my guts again. I was the only guy at the bar with a bullet in their guts. I kept putting my hand under my jacket, on my stomach and all, to keep the blood from dripping all over the place. I didn't want anybody to know I was even wounded. I was concealing the fact that I was a wounded sonuvabitch.
There is an interesting contradiction here. Holden is concealing "the fact" that he is wounded, but this fact is itself a fiction, since he is not wounded. He is hiding something which is not really there. His drunken state prevents him from analyzing the irony of this, as he certainly would if he were sober. However, Holden does feel wounded mentally and emotionally, just as he did after the episode with Sunny and Maurice. He uses the "bullet in the guts" as a metaphor for the real pain he feels, and which he is trying to conceal from everyone, at the same time as he wishes he could find someone to understand his anguish. Holden is awkwardly poised between childhood and adulthood, and his fantasy life here reflects the awkwardness of his position.