As a follow to the article on direct impingement, this article discusses the use of a piston driven gas system. A piston system is used as a replacement for DI because gas pistons are not recycling "dirty" gas back into the weapon like the DI system. The gas that is recycled with a DI system contains carbon and small amounts of unburned gun powder. Over time, this will collect on all the moving parts inside the receiver and cause the weapon to malfunction.
The piston system will still use the gas from a fired cartridge. However, it uses that gas to operate a piston which then cycles the weapon without introducing dirty gas to the receiver parts. This allows the weapon to operate for longer between cleanings and prolongs the life of internal components. Some of the piston systems have different pressure settings for shooting with suppressors. Piston systems are quickly becoming the standard on all gas blow back systems. They are found on AR weapons from LWRC, Ruger, and the kick-ass ACR. The video below demonstrates the difference between the two gas systems. I think the video is somewhat biased on the piston system because it shows the DI system failing. Both systems are very reliable, and the DI system has been used by the US Military for decades. Do not judge the reliability of the DI system by this video alone.

The AR-15 operates using a gas blow back system. This means that as the weapon is fired, some of the gas from a fired cartridge is recycled before leaving the barrel. It travels down a tube which blows the gas back into the receiver to eject the empty cartridge. It is called a cycle. This all happens in less that a second and is vital the weapon operating correctly.

The traditional gas system explained above is called Direct Impingement or DI for short. Most moderately priced AR-15s will use a DI gas system. The complete operation is illustrated below:


The gas system on an AR-15 can be adjusted for various situations or combinations of equipment and accessories. This is usually done with a special upper receiver or gas block. Some gas blocks are user adjustable via a screw or knob. This allows the amount of gas to be adjusted for use with suppressors or different barrel lengths. Shown below is a good example of tuning a gas block to allow the weapon to cycle properly:


There is also another article which discusses the use of piston operated AR-15 gas systems.

Buying a used gun can be tricky. Most of the time, you do not know the previous owner or what has been done to the gun. Used guns may have been modified in ways that make them illegal or otherwise unreliable. Some key parts of the gun may have excessive wear which can make the weapon less accurate or cause various types of malfunctions.
If possible, always shoot a used gun before purchasing it. Most gun shops will let you fire the weapon if you indicate a genuine interest in the gun. Try to fire at least 2 magazines, and try to shoot quick bursts or rapid fire. What you should look for is any problems with the feeding or ejection mechanisms. This is especially important with handguns and assault rifles which rely heavily on blow back or roller locked design. If you encounter any type of malfunction while firing two magazines it should be a red flag.

Things such as recoil springs and hammers should lock up tight and strike with plenty of force. Recoil springs and firing pins can be replaced and are required to be replaced after a certain number of rounds. However, replacing parts on a used gun is like putting money into a used car. If you are buying it used, you probably don't want to spend more money to make it function perfectly.

Barrel use can be loosely determined by the barrel rifling. Over time, a barrel will show signs of wear by a decrease in visible rifling. This will significantly reduce accuracy at long distance. Some hand guns (like the H&K USP) may show wear on the barrel hood which can provide an indication of how much it may have been used.

You should expect cosmetic imperfections on used guns. Things like holster wear, nicks, and scratches will all happen once a gun is used or handled. You should not be concerned with such things unless you are seeking the gun for a collection. Keep in mind that guns are fired at least once before leaving the factory so they may appear used even if they are advertised as used.