The targets are the size they are for a reason. And so are the scoring rings. The D target isn't the only target the military shoots at 300. (I don't know where you came up with the "A" Target, but it looks just like a SR target which has an X ring of 3" and a 10 ring at 7"). The goal of putting them in the X & 10 rings of an SR is obviously training for hitting head-sized objects.
Even your picture shows a transition from head/shoulder shots to body shots at 500.
