[Klaus]

In general, longer barrels give you more power, more accuracy, and less kick. They also make the gun bulkier, heavier, and slower to draw. Police guns are either revolvers or semiautomatics. A revolver has a cylindrical peice that has chambers that hold the ammunition (usually 6 rounds); it is called a cylinder. Semiautomatic pistols hold ammo in a spring loaded case, called a box magazine; many people mistakenly call it a "clip". A semiautomatic firearm shoots 1 round with each trigger pull, then uses some of the energy to eject the empty case, cock the hammer, and load a fresh round, so it is ready to shoot again.
Pistols can have two types of triggers, Single Action and Double Action. In single action, the trigger only does 1 thing, it releases the cocked hammer or striker to fire a shot. A Double Action trigger does two things, first it will cock the hammer (if it is not already cocked), then it will release it. On a double action semiautomatic pistol, normally only the first shot is DA, since the gun cocks the hammer after the shot, so the rest are SA. I hope this is enough info to get you going.