So. A McCree manages to get line of sight on 6 players who all happen to have their sound turned off at the same time. What order do they get punished in? The first that came into McCree's line of sight? The one with the highest high noon charge? From left to right? Random?
McCree fires right to left, always. It does not matter if you acquire more right targets after your left-er targets. It also does not matter which shots will result in kills -- right to left, always.
This can be verified for yourself in the Practice Range, where there are 2 Training Bots next to each other.