Spring
Spring types airsoft guns are the most basic guns. Most of the time, they are found as either pistols or snipers. The basic principal for how a spring type gun works is the same even if it is a sniper or pistol. Because of how basic they are, they typically considered some of the most reliable types of guns. Generally, the more parts there are, the more likely one is to break. Spring guns have minimal parts ensuring there is less likelihood of any of them breaking.
How they work
When you pull back the cylinder it compresses the spring and piston which is grabbed by the trigger sear. When you pull the trigger sear, the spring launches the piston forward creating highly compressed air in the cylinder which pushes out through the cylinder head and hits the BB. When the air pushes the BB, it strikes a rubber patch called the hop up bucking which causes a backspin while it flies. The photo shows a good visual of the process. While the photos may be a bit simplistic, they represent the general idea
Pros
There are many great pros with spring guns:
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Work well in any climate conditions
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Easy to maintain
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Few parts to upgrade/replace
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Can get better range than AEGs or gas guns
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Can handle heavy weight ammo better than AEGs or gas
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Quiet
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Reliable
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Low ammo consumption
Spring guns work well in any climate conditions because they do not rely on a battery (which can get very temperamental in cold) or gas (which compresses more in the cold and become less efficient). They are easy to maintain because when you get a good setup, they generally don't break. As shown in the graphic, spring guns have few parts, at least fewer than AEGs. Spring guns get better range due to their ability to have a very high cylinder volume, that is also the same reason they can handle heavy weight ammo better. They are also quiet because there is no moving motor or gears, the only noise emitted from these types of guns is from the impact of the piston onto the cylinder head. They are also very reliable due to the few parts. Due to the fact that these guns are only bolt or pull action, they use a very low amount of ammo
Cons
Spring guns are not without cons though:
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Can be hard to find high quality upgrade parts
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Limited to single shot
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If a heavy spring is installed, it can be hard to engage the shot
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High quality upgrade parts are very expensive
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Heavy weight ammo is expensive
Spring gun owners are a highly specialized market which makes it tough to cater properly to them. Most companies don't try to. It also doesn't help that once they get a high quality part, they are not likely to need another one for a very, very long time. Spring guns require the manual labor of the user to engage the shot, which means that you cannot use a spring gun in a semi automatic or fully automatic fashion like AEGs or gas guns. On the same note, if the user decided to install a very heavy spring (170-210) then they can be very hard to pull back. Spring guns are often very expensive to use, both for parts and ammo. Since spring guns are generally snipers, they mostly use very heavy BBs which are far more expensive than low weight ammo.
Summary
While spring guns would make horrible assault rifles, they are perfect for snipers. Almost none of the cons matter to snipers due to the way they use their guns. The simple fact that spring guns are reliable, consistent, and adaptable make them the perfect sniper rifles. This can sometimes also be true for pistols. While generally pistols are gas, spring pistols are sometimes used by snipers for the same reason that they use spring for their rifles. If a sniper needs to make a quiet, up close kill without giving away their position, a spring pistol will work better than any gas pistol would.