Because it is what was taught in basic, many shooters assume that “center of mass” is the optimum approach to bullet placement. It isn't.
Firstly, “center of mass” is something of a misnomer. What we are actually attempting is placing the bullet in the centre of visible shape. While the term “center of mass” is freely used, it is seldom defined. I think of it as the centre of an X drawn from shoulders to hips, but I suspect others may use different visualizations.
If you have learnt a little anatomy, such as reading “Attack, Avoid, Survive”, you will understand that putting a bullet into the centre of shape will often avoid hitting the central nervous system unless the enemy is running straight at you.
Often you will not have a shot at the torso. When a head appears around cover, firing at its centre will often result in a miss. A better point of aim is about an inch below the visible area.
Center of mass does have its uses. It is taught since it is felt to be easy to learn, and it is because it is what we have always done. Against a vehicular target, center of mass (or leading edge) is a good aimpoint. If you are springing an ambush, chaos and disruption are primary objectives. Multiple wounding shots or near misses may be more effective in that context than a lesser number of clean kills.
In “Attack, Avoid, Survive” and “Survival Weapons” I explained shot placement in the context of anatomy. If you have a relatively good view of your target, or sufficient time, this is your best approach.
The following two illustrations are of interest:
The first is taken from a WW2 manual for American snipers (FM21-75 Feb, 1944 p.172). The rifle was zeroed to 400 yards and the shooter encouraged to use offset aiming rather than adjusting their sights. Note that at 400 yards the intended target appears to be the armpit-level line, as advocated in my own books. At less than 400 yards, the sniper is recommended to aim twelve inches below the intended point of impact.
The second illustration is from a manual for the M14, which was zeroed to 250 metres. At ranges of less than 200 metres the round would hit high so soldiers were taught to aim at the bottom edge of the “center mass”.
Most military rifles are zeroed to 300 yards or metres. Some older models have battle sights set for 400 yards. Yet most combat shots are made at less than 200 metres, where the bullet is expected to hit several inches above the point of aim! Any wonder that shooting directly at a face will so often miss?
In combat, it is common for troops to shoot high anyway. This is partially stress, but also poor visibility makes targets appear more distant. Differences in elevation will also have an effect.
Firing through a sloped windscreen will tend to deflect a bullet upwards. This occurs if outside firing in or inside firing out. The solution is to aim low.
Often a target will be at a higher or lower elevation. You may be firing down from a hill, or being fired upon from an upper window or roof. The actual range to the target is not a straight line between the shooter and target. Imagine a right-angled triangle, with the shooter at one corner and the target at the other. It doesn't matter which is higher, since the effect is exactly the same both “uphill” and “downhill”. The direct distance between shooter and target would be the hypotenuse of the triangle. As far as the bullet and gravity are concerned, the relevant distance is horizontal, the length of the triangle base. The true or horizontal range will always be shorter than the slant range. Using a 5:4:3 triangle for illustration, the horizontal distance will be 20-40% less than the direct line between target and shooter. Shots up or from elevations tend to hit high.
There are two solutions to these effects. If you have any choice in the matter, zero your combat rifle to 200 metres so that it tends to hit what you point at. This gives a mid-range trajectory of only two or three inches. For longer ranged shots, learn the correct holdover and offset aim-point. Tactically, you are often better off waiting for an enemy to get close or pass by. Long-range engagements are better left to machine-gunners, mortars and snipers.
The second solution is to make the belt-buckle your default point of aim. I believe there is an episode of the Simpsons where Homer claims the family moto is “Aim Low! Avoid Disappointment!”.
• If you are at a higher or lower elevation, aim at the target's belt-buckle.
• If you are uncertain of the range, aim at the belt-buckle. A short shot may still glance off the ground and hit the target.
• If the target is moving, visualize the belt-buckle and aim for it. This method automatically tends to adjust for relative angle of motion. If the target is moving obliquely aiming for the buckle will put less lead on the shot.
• Unless range is very short, snap shots should be aimed at the belt-buckle.
• A target may be prone, or looking around cover. Aim your shot about an inch below the visible target area. A low shot may still endanger the target.