In our previous post, we saw how an user can use the scope to estimate the range to a target. Once the range is estimated, the user can adjust for windage and elevation. As a matter of fact, most telescopic sights have controls to adjust for these parameters. Since some scopes have variable magnification, these may have additional controls to zoom the view and adjust for parallax error. Some scopes may offer an illuminated reticle for low light conditions and have an additional knob to adjust the brightness. Some very high end models even come with built in laser range-finders for better accuracy.
Since very few weapons are designed with built-in telescopic sights, most of these are mounted separately. Most western military rifles are designed with a standard picatinny rail on them, to which various accessories (including a telescope) may be mounted.
M4 carbine with scope mounted on picatinny rail on top of the weapon. Click on image to enlarge.
In other weapons, people usually attach a scope base to the rifle and then add scope rings to hold the scope in place on top of the base.
There are some advantages to telescopic sights. In most iron sights (except aperture type iron sights), the user needs to focus on three points simultaneously, which is not easy to do. With a telescopic sight, all the user needs to do is focus the cross hairs of the reticle on to the target. With a magnified view, it allows for shooting at longer ranges. The shooting as well as target identification are more precise as well. They also provide much more accurate windage and elevation measurements than other systems we've studied so far. Most modern high quality scopes are also surprisingly durable and can take a fair bit of punishment.
There are also some disadvantages. The main one is cost. Telescopic sights are way more expensive than iron sights, though the cost has come down in recent years. These sights are also more bulkier than iron sights and definitely alter the balance of the weapon. Thirdly, they cannot withstand bad weather as easily as iron sights. Also, some weapons that eject cartridges from the top (such as lever action rifles) need to have the scope mounted slightly off center, so that it does not interfere with the ejected shells.
On heavy recoiling weapons, the scope rings need to be tightened very consistently, otherwise the scope will go out of alignment.
For a long time, most militaries would only equip snipers with telescopic sights, because of the high cost of a scope. However, as the cost of scopes has fallen in recent times, some militaries have started to equip some regular infantry with scopes as well.