I copied this from my TLDR post on choosing an airgun. I thought this might be useful on its own . . .

HOW DO "AIRGUNS" work?

“Airgun” is actually a bit of a misnomer. The term generally applies to three types of powerplants: spring, gas and air.

Spring guns – a spring gun doesn’t actually produce “air” until the moment the trigger is pulled. Imagine a syringe, the kind the nurse uses to give you a shot. It is a cylinder with a hole on the business end and a plunger (with a tight seal) on the other end. When the plunger is pressed forward, whatever is in the cylinder comes out the front. Now imagine a metal cylinder with a sealed piston at the back end and an air hole (and rifled barrel) on the other end. At rest, the piston of a spring airgun will sit forward in the cylinder, thanks to a massive coiled spring placed behind the piston. Cocking the gun (by whatever means) pushes the cylinder backwards, compressing the spring, until a hook on the back of the piston mates with a corresponding hook on the gun’s trigger. Placing a pellet in the back of the barrel creates a closed airspace between the retracted piston and the rear of the pellet. Trip the trigger and the spring releases, causing the piston to fly forward, compressing the air in the cylinder until the rising pressure overcomes the resistance of the pellet in the breech and the pellet goes flying down the barrel and toward the target. Simple, huh?

The $29 Daisy Red Ryder you used as a kid was a spring gun. The $500 Air Arms TX200 that dominates the hunter class of Field Target airgun competitions is also a spring gun.

The main advantage of spring-powered airguns are that they can produce a fair amount of power in a relatively compact and COMPLETELY SELF-CONTAINED package. If you’re a squirrel hunter, you can go into the field with your spring rifle, a tin of pellets and you’re good to go. You can shoot until you run out of pellets with no concerns about running out of power or carrying extra do-dads with you.

Spring airguns are simple, robust and reliable. With minimal maintenance, a good spring airgun will shoot for decades, and actually get smoother the more you use it.

The main disadvantages of the spring airgun is all the twang and vibration that happens when that big spring releases, sending the piston flying forward. Spring guns move around in your hands when you fire them, forcing you to concentrate on proper hold (don’t grip the forearm) and follow-through. They’re not easy to shoot well. They’re also hell on scopes. Depending on power levels, they can require a fair amount of effort/strength to cock.

Gas guns – For all practical purposes, modern gas guns are all powered by carbon dioxide (CO2). A charge of carbon dioxide waits at the ready in an enclosed reservoir. When the shooter trips the trigger, a valve is opened for a split instant, releasing a measured charge of gas, sending the pellet down range.

Most of us are familiar with the little CO2 cartridges that were first used in self-inflating life jackets and to create soda water behind the bar. Who knows how many little 12-ounce-gram  CO2 powerlets have been consumed powering all the different cheap pellet guns that Crosman and other companies have sold over the years? But there’s more to CO2 guns than the myriad firearm look-a-like pistols sold at Walmart. There was a time when some of the finest match-grade pistols and rifles were powered by CO2, though most of these were fueled by larger bulk-fill tanks. But bulk-fill CO2 didn’t begin with Olympic-grade guns. Back in the 1950s Crosman offered a really neat line of bulk-fill pistols and rifles that are still valued by collectors and airgun enthusiasts today. They even came with a 10-ounce steel tank that could be sent back to Crosman for a refill when the owner ran out of juice.

Today, C02 guns run the gamut from firearms clones, to design-your-own single shot pistols and rifles sold through the Crosman custom shop. In fact, customizing and modifying CO2 guns is a vibrant and interesting segment of the airgunning hobby, with lots of cottage shop suppliers offering custom parts and do-dads to individualize these guns.

The main advantages of CO2 guns include ZERO recoil, relatively low-cost and the ability to cram the mechanism into almost any shape and form of gun (thus all the look-a-likes). Also, unlike spring guns, it takes almost no effort to cock a CO2 gun, a characteristic that makes them good for kid’s guns. Also, because not all the CO2 is dumped when you pull the trigger, follow-up shots are immediate and repeater designs are possible. In fact, Crosman made a successful semi-auto CO2 repeater as early as the 1960s.

Disadvantages of CO2 are many. Because of the physics of compressing carbon dioxide, there is a practical limit to the amount of power that a CO2 can produce. They are best for low-to-moderate powered guns. They’re also loud (almost as loud as a .22 short). Finally, carbon dioxide freezes. These guns don’t work well in cold weather and rapid firing can cause them to lock up. Also, CO2 costs money and airgunning is supposed to be cheap. Other than the classic Crosmans from the '50s and '60s (which I love) and the modern Crosman based Tinker Toy projects, I can't see recommending a CO2 gun for most users.

Pneumatic guns – These are air guns in the truest sense of the word. These guns contain an air reservoir. When you pull the trigger, a valve opens momentarily and the resulting burst of air propels the pellet. There are three basic forms of pneumatic guns: single-stroke, multi-stroke and pre-charged.

With a single-stroke pneumatic air gun, you operate a lever (usually) ONCE to charge the air reservoir. That’s all the power you get. When you pull the trigger, all of the air is released and the gun must be recharged for the next shot. Single-stroke pneumatics run the gamut from cheap kid’s guns to high-grade match guns (though they’ve largely gone out of favor in competition).

Multi-pump pneumatics allow you to vary the power of the gun with subsequent pumps. The Crosman 760 you had as a kid was a multi-pump pneumatic, as was that fancy Benjamin or Sheridan the rich kid down the block had. Every kid knew two or three pumps was good for quick shots at plastic army men, but that a full charge of 10 pumps would practically rival the awesome power of G.I. Joe’s Colt .45 (or so it seemed). In truth, guns like this are fairly powerful, and like the spring gun, are completely self-contained. You can go to the field with your pump-up Sheridan, a tin of pellets and shoot all day. Like the single-stroke pneumatic, tripping the trigger releases ALL of the air in the gun. You have to pump some more before you can shoot again.

While multi-pumpers like the Sheridan and Benjamin are handy rifles, they don't scope that easily. There are decent aftermarket mounts for them but you're still faced with variable point of impact, depending on the number of strokes.

Pre-charged pneumatic airguns (PCP) have an air reservoir that must be filled from an external source, such as a hand pump, a larger air tank or a compressor. Unlike the multi-pumps that dump their entire air charge each time the trigger is pulled, the valve of a PCP airgun releases a measured/regulated burst of air, leaving the rest of the air in the on-board tank for subsequent shots.  Not only are rapid follow-up shots possible, but the pre-charged pneumatic design lends itself to repeaters.

The advantages of pre-charged pneumatics are many. Unlike spring guns, there is virtually no recoil and absolutely no spring twang. This makes a PCP much easier to shoot accurately and there’s no worry about trashing scopes. Also, there really is no practical limit to the amount of power a PCP airgun can produce. You’ve seen those massive pumpkin-chunking cannons on TV? Yep, precharged pneumatics done large! Those .40 caliber airguns that guys are killing wild boar with? Yep, PCP. The possibilities are nearly limitless.

BTW, this is not new technology. Lewis and Clark carried a large bore, 22-shot airgun on their 1804 Voyage of Discovery. The gun featured an air reservoir in the butt that was charged with a hand pump. They used it to amaze the Indians and scare them into submission. Powerful magic, those PCP airguns!

Pre-charged pneumatics are not without disadvantages, though. First, they’re expensive. The popular and very good 10-shot Benjamin Marauder broke a huge price barrier when it was introduced at about $450. (Great  -- read: European -- PCP repeaters could be 3 times that money.) That’s just the gun. Add a scope, rings, a hand pump or some sort of air tank and you could easily double the price. They are also definitely not self-contained. Run out of air in your on-board tank and you need to hook up to your SCUBA or SCBA tank or start pumping that hand pump.

I've been thinking about an air gun of some sort.Good read ...thanks!

There can be a bit of twang and vibration from a PCP, just a lot less.  Most PCPs use a striker to open the valve (yes, some call them hammers but a hammer swings, a striker slides) and the striker is spring propelled.  



The difference between a springer and a PCP in this twang/recoil is several orders of magnitude.  Which makes the PCP much easier to shoot.



If you go with hand pumping a PCP, humidity can be a real problem.  A few hand pumps have a desiccant filter on the inlet to reduce this problem.  Other pumps can be retrofitted to have one, some home made options are out there.  Or you can feed the pump dry air from a tank containing desiccant.   This is what I have done, using a pressure regulator set to 20 PSI.  This greatly speeds pumping plus I no longer have water in my pump or rifle.



Anyone with a shop air compressor knows the tank collects water and must be drained.  This happens because air cannot hold as much water vapor per volume when compressed.  So even if you are living in Tuscon AZ, you can still have water condense out in your pump and rifle.  And water in the pump ruins the lubricant (silicone grease).


 

Thanks Keith. My knowledge of PCP's is mostly theoretical.