Two words: Vertical Track.
Lethality is a complex equation however at my school what we teach for 1v1 engagements is to "track" your target with 4+ rounds from the belt buckle to the top of the head, up through what is called the "cone of vulnerability". It's been proven that the greater dispersion of impacts is far more effective of shutting down the human body than pairs of shots into the same area.
Pretty much every school is teaching it, and it gives you a lot more chance of a round hitting something important like the spine, lung, heart, medula etc...this balances out the need for precise shot placement, particularly in close quarters when time is not on your side and you are using an "unsighted" firing technique.
Hey Lazs, can you own the 4" Smith 500 down there? I'd love it if I could get my hands on one, but it's 1/4 inch too short to import into Canada.