 Assuming McClain does get the first shot and does shoot Dunn in the head, knocking him out, McClain would assume Dunn was dead. I certainly can't see him deducing that Dunn is bulletproof fast enough to restrain what should be by all rights a cadaver and mount some kind of grotesque, unMcClainesque series of test. Dunn actually recovers fairly quickly from a knock out. I mean he got up in less than a minute of the car crash and a bullet would be slightly less impact wise. 