SA means single action. DA stands for double action. This refers to the function of the trigger in regards to hammer cocking. The 1911 Government .45 pistol is a SA because the hammer must be pulled back by one's thumb or by chambering a cartridge before the trigger serves its purpose. The Beretta 92 9mm pistol will fire with the hammer from its fully forward position just like a modern Smith&Wesson revolver. After a round is fired from the .45 the hammer will remain cocked until it is slowly lowered with the thumb while pressing on the trigger or the trigger is pulled without a live round in the chamber. The Beretta, like most modern DA auto pistols, has a safety which lowers the hammer safely for you when applied. The SA auto pistol does not lower the hammer but the hammer is prevented from movement by the trigger while the safety is on. Some pistols are DAO or Double Action Only. This means the hammer is safely lowered with the return of the slide for every shot. Glocks are an example of a striker fired pistol. There is not a traditional exposed hammer on the Glock. The safety is mounted on the trigger and is considered passive. The trigger pull is shorter and lighter than a true DA. The trigger pull also remains the same during firing if you let the trigger fully return. The Glock is not a real single action either because the hammer is partially cocked by the action of the trigger I believe. All three of the trigger actions have advantages and personal choice and the intended role of the pistol play a big part in picking the right one for you. I have mainly described these terms as they are used in semiautomatic pistol terminology. This is how they are most often used. However, SA, DA, and DAO can refer to revolvers as well. If you're interested in discussing the advantages and disadvantages of each action, I'm sure you will get plenty of responses. The Beretta 92 9mm pistol is also refered to as a DA/SA pistol which means the trigger only needs to pull back the hammer on the first shot . After the slide cycles from discharging the pistol, the hammer remains cocked until it falls on a defective cartridge which fails to fire or the hammer is manually lowered. The trigger pull becomes shorter and lighter after the first shot just like SA. This DA/SA action is the traditional method as opposed to DAO of double action pistols. Hope that helps clear things up.
[This message has been edited by JackNKoch (edited November 15, 1999).]
[This message has been edited by JackNKoch (edited November 18, 1999).]