In the beginning there were no firearm sights. It didn't matter. Firearms of the day were not very good at sending projectiles in the same direction time after time.
Sights have been around a while, probably since the 1500s. Hey, if it made them feel better, who am I to argue? The fact is, you can't reliably hit anything at any distance with a smooth bore.
Then along came rifles. Spinning bullets go where they are pointed. But where is that exactly? Looking down the barrel in the general direction of your target is not good enough for rifles.
Open sights, also called iron sights, are fairly simple. A post or ramp on the front of the barrel is aligned with a notch at the rear of the barrel. This works very well in the hands of a skilled shooter with good eyesight.
Above: Four variations of open sights. Black= Rear Sight, Yellow = Front Sight, Red X = Bullet strike.
Three dot sights (far right) often have luminous dots for low light shooting.
Peep sights also have a post at the front, but they have an aperture in the rear. The top of the post is centered in the aperture. This is an easier system in some regards because you don't have to work as hard to maintain sight alignment.
Many military rifles have peep sights.
A Globe sight is basically a tube mounted on the front of the barrel, with interchangeable sight elements. This allows you to have an aperture at the front and at the back. These work well with targets that have a high contrast circular center. Peep through the rear aperture and place the front aperture concentric with the target center. It turns out that human eyes are real good at concentric.
Reflex sights use a partially reflective optical element to project an aiming reticle, often a red dot, into the eye of the shooter. This is also how heads-up displays in aircraft are done.
To the shooter the dot appears to be on the target. Since there is only one aiming element in this system, sight alignment is not a factor.
Some reflex sights look like mini telescopes, others are not much more than a window-like glass element. Most do not magnify but some do.
Reflex red dot sights rule in pistol competition.
Pretty simple. A laser attached to the gun puts a visible dot on the target.
The laser might be an add on, or it might be built into the gun. Grips are a popular spot for lasers.
Lasers might be hard to see in bright light or at a distance.
Telescopic sights, or scopes, put an aiming reticle of some kind, usually crossed lines called cross hairs, in the focal plane of a telescope mounted to a firearm. The reticle then appears to be on the target.
Telescopic sights have the advantage of making it easier to see your target.
There are many variations of telescopic sights.
Not all firearms have adjustable sights. Firearm sights that are not adjustable are referred to as fixed sights. Sometimes fixed sights can be replaced with adjustable sights. Just about any gun can be modified for adjustable sights.
The directions of sight adjustment are windage (left/right) and elevation (up/down). Most often it is the rear sight that is adjusted, although the front sight might be adjusted to get you in the ballpark or to get the rear sights in the center of their range under average conditions.
On the target to the left there is a group of three shots in the lower left quadrant. To get these shots to the center of the target they must move up and to the left. Therefore the rear sight must be moved up and to the left.
Sights are often adjusted with screws or knobs. Most are marked to indicate which way to turn the screw or knob to move the sight in the desired direction.
Sight adjustments are often talked about in minutes of angle, or minutes of arc (MOA). There are 360 degrees in a circle and 60 minutes in a degree. (A minute of angle is divided into 60 seconds of angle but this is too fine a division for shooting.)
Think of this angle as a long skinny piece of pizza. We shoot from the pointy end. What we want to know is how wide is the piece of pizza at a certain distance from the pointy end. Well, it turns out that a slice of pizza that is one minute of angle, 1 MOA, is one inch wide at 100 yards from the pointy end.
(For all you math whiz people it is actually 1.047" at 100 yards. If you need to worry about that extra .047 inches, I want to know who makes your rifles.)
This one inch scales in direct proportion. In other words 1 MOA = 2" at 200 yards and 5" at 500 yards and .5" (one half inch) at 50 yards.
So, looking at our target again, let's say that we shot that target at 50 yards, and let's say that the shot group needs to move 3" to the left and 2" up. We need to adjust the rear sight 6 minutes left and 4 minutes up.
If your sights are calibrated in minutes this will be easy. Many adjustable sights have "clicks", discreet adjustment increments that make an audible click sound. The clicks can be 1 MOA or 1/2 MOA or even 1/4 or 1/8 MOA. You have to know how your sight is calibrated.
But what if your sights are not calibrated? Simply compare the distances that relate to the target with the distances that relate to the gun.
One distance that relates to the gun is sight radius. Sight radius is just the distance between the rear sight and the front sight. Let's say it is 20". The other distance that relates to the gun is how far to move the sight from side to side. This is what we are trying to figure out.
First let's get everything in inches. 50 yards x 36 inches per yard = 1800 inches. The group needs to move 3" left. So 3"/1800" = x/20" where x is how far to move the sight.
Remembering back to the time Sister Mary Peter spent about half of the 1960s trying to teach me to solve problems like this, I multiply 3 by 20 and divide by 1800. So 3" x 20" / 1800" = .033", meaning your rear sight has to move .033" (about 1/32") to the left.
You could instead move your front sight .033" to the right. Front sight adjustments are always in the opposite direction of what a rear sight adjustment would be.
Note that a smaller sight radius will lead to a smaller sight adjustment. The .033" adjustment for our rifle with the 20" sight radius would be .0067" for a pistol with a 4" sight radius. Looking at that from a sight alignment point of view, if you misalign the sights on that pistol by just the width of a human hair, your shots will be 2 1/4" inches off the mark at 50 yards. Long sight radius is a real advantage with these kinds of sights.