He's crucially aware of his breathing rhythm, because he wants to fire between breaths. He probably doesn't think much about trigger pull. He wouldn't be here if he didn't know how to pull a trigger. He's not 'pulling' it in the sense of exerting his muscle against it, so much as urging it to cooperate it, massaging it into doing his bidding.

Is it just me, or did it suddenly get really hot in here?