A M16A2 and two M16A4s. Click on images to enlarge. Public domain images.
In all these examples, note that the sights are a good 2.5 inches from the top of the barrel. Why is this the case with the M16 family? We will study the reason in this post.
The first thing to notice in the M16 family of firearms is that the stock is in line with the center of the barrel. In most other rifles, the stock is a little below the line of the barrel and they have small sights mounted on top:
Click on images to enlarge. Public domain images.
The reason why the stock is below the barrel is for ergonomics. When the user rests his/her cheek on the comb of the stock. the user's eye is aligned just above the top of the barrel and therefore the sights can be placed right above the top of the barrel for such firearms.
However, for such stocks, when a bullet is fired, the recoil pushes back in the line of the barrel and the resistance offered by the user's shoulder is below the line of the barrel and this creates a rotational torque. We studied this effect when reading up on muzzle brakes and compensators earlier:
Here, A is the force acting because of the recoil and B is the force resisting the movement from the user's hand and shoulder on the grip and stock. Since A is higher than B, this causes the rotational torque C, which causes the front of the barrel to rise when shooting. This can cause a loss in accuracy, especially when firing rapidly.
In the M16 family of rifles, the rotational torque is minimized by placing the stock higher, so that forces A and B are around the same line with each other. This is called a "straight line layout" and is done in order to make the rifle easier to control, especially when firing in burst fire or automatic fire modes. One more nice feature of the straight line layout is that it allows the operating rod and buffer to run directly back into the stock and thus reduces the overall length of the rifle. However, with a straight line layout, the user cannot comfortably aim the rifle if the sights are just above the line of the barrel. Hence, the solution was to make the sights taller on the M16 family, so that the user can place their cheek on the stock and still look through the sights comfortably.
By the way, the M16 isn't the only rifle that does this. The German FG42 and MG42 and the US M60 are three earlier examples that have tall sights as well:
FG42 and M60 machine guns. Click on images to enlarge. Public domain images.
In the above images, note that they all have a straight line layout. The FG42 was one of the first firearms to have this feature and came out during World War II. The straight line layout was later adopted by others, such as the M16 family, the Russian Dragunov rifle, the German Heckler and Koch HK36 and XM8, the Swiss SIG SG 510 etc.
A Swiss SIG SG 510. Click on image to enlarge.
Licensed by user Rama under the Creative Commons Attribution-Share Alike 2.0 France license from wikipedia.org
With taller sights, the user can rest his/her cheek on the comb of the stock as usual and still aim comfortably through the sights. This is why all rifles with straight line layouts have taller sights.