Yeah, full-auto would have been a waste but semi-auto would've been more useful. You aim, shoot, then move to the next target mostly by just moving the gun laterally a bit. (Since all the heads would be virtually on the same plane.) But with the single-action gun he had you have to shoot, stop and lower the gun to re-cock it, re-raise it and the re-aim from scratch.
Semi-auto could've made some difference, not likely enough to change things in the gun's favor but it'd net different results.