Choosing the correct type and variation of site is the most important decision you can make when setting up your gun. Many factors should be considered in deciding on a siting system for your rifle, shotgun or handgun.
The type of shooting you will be doing will guide your decision on whether to purchase a reflex site, scope, dot site or open/iron site for your gun.
Range: The typical distance to your target
Acquisition Time: How much time will you have to put the gun on target and get the shot off
Shooting Conditions: How bright/dark will the surroundings be? Will wind play a significant part in your shot? Will the target be in forested or open terrain?
Budget: The difference in a $100-$400 scope can be significant, while $400-$1600 offers less of a return. That being said, if you commonly shoot at very long distances or in competitive matches, the money is often worth it.
Types of Sites
Open/Iron Sites: Standard open sites (bead, fiber optic, A2 front/rear, flip-ups, ghost ring). These sites will work at any reasonable distance, but are preferred for closer than 100yards. Anything longer and shot placement may suffer. The sites offer quick acquisition of the target and have no electronics to fail or require batteries.
Reflex Holographic Weapon Sites:Reflex sites offer quick target acquisition at ranges up to about 50yards. They tend to come unmagnified, but their range can be extended with the use of tactical magnifiers. These sites work by projecting a laser image on a pane or panes of glass. Common reticles for reflex sites are the dot-cirle, cross, dot and various hybrids. The better sites are error corrected (aka parallax-free) meaning that even if the shooter isn't directly behind the sites, the reticle will indicate point-of-impact. The shooter should aim with both eyes open allowing for quicker aiming and the shooting of moving targets at close range.
Dot Sites: Referred to as Red Dot or Green Dot sites, these sites are typically in a tube similar to a small scope. The tube configuration can help in very bright situation. Dots are another type of holographic site as they project a single dot on a glass pane. These sites are often offered in multiple magnifications from 1x up to 4. These sites can also benefit from the use of tactical magnifiers.
Scopes:Scopes are typically used at longer ranges (100+ yards). They are offered with a multitude of reticles from simple reticles, rangefinders, mildot to wind and ballistic drop compensating site pictures. While effective at longer ranges, scopes suffer at close range. The magnification can make it difficult to acquire a target quickly at 0-50yards or to stay on a close-up quickly moving target.