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I have noticed this question in search terms that have brought people to the blog recently. Here’s a concise explanation. A double action pistol cocks its hammer when you pull the trigger. A single action does not. Almost all modern revolvers are double action and single action together, meaning that they will cock their hammers when you pull the trigger or you can thumb cock the hammer and use the gun in single action mode. The classic cowboy revolver, the Colt Single Action Army revolver (the one all the heroes of the silver screen carry) is a single action revolver. It can only be fired if you manually cock the hammer first, usually by pulling it back with your thumb or “fanning” it with your other hand.

Most modern autoloading pistols are either double action only or double action/single action. This distinction is technical but it’s important. Guns like the Glock, the S&W M&P, the Kahr and similar guns are double action. The hammer or striker is cocked by the pull of the trigger (or partly so in the case of the Glock and some others — part of the cocking is performed by the rearward motion of the slide) and every trigger pull is the same. The best known example of the double action/single action is the Beretta 92F (US Armed Forced designation M9). With this pistol, the first trigger pull is double action, but after the first shot is fired, the slide cocks the hammer and subsequent shots are single action – you don’t have to pull the trigger as far to get the gun to fire and the trigger pull is considerably lighter.

The greatest of the autoloaders, IMHO, is neither double action nor double action/single action. It is the Colt M1911A1 .45 Automatic and it is pure single action. For this reason, this pistol is carried “cocked and locked” meaning that the hammer is cocked and the manual safety is applied. The reason for this is that the M1911 will not cock the hammer when the trigger is pulled. It must be cocked for the trigger to work, either by the action of the slide or by thumb-cocking it. When you load an M1911, you insert a magazine, rack the slide thereby cocking the hammer, and apply the manual safety. This scares the hell out of people who don’t understand them, but it’s actually quite safe. The legendary Browning HiPower 9mm is also a single action pistol. The Springfield Armory XD series of pistols are also single action. They are cocked completely by the movement of the slide.

What are the advantages and disadvantages of each:

The double action trigger pull is long and heavy, typically requiring 8-12 pounds of pressure to fire. This is basically a safety feature. You are less likely to accidentally set off a round when the trigger pull is long and heavy. But a long and heavy trigger pull usually means slower speed and less accuracy because of the muscle tension required to operate the trigger.

The single action trigger pull is very short and light. Typically 3-6 pounds of pressure is required to fire. In revolvers, it is even lighter than that. The lighter, shorter single action trigger pull translates to greater speed and accuracy. The downside is that the single action is unforgiving with blunders. People who use these guns need to practice a lot and become intimately familiar with their operation.