When people talk of guns, they often refer to them by the action the gun uses to prepare the next round to fire.
For centuries, all guns were single shot weapons. You’d fire once and then have to reload. If you wanted to fire more than once, you needed to add another barrel.
Until the metal cartridge came along, all guns were muzzle loaders. That means that the powder and bullet were shoved down the front of the barrel. The “primer” may have been a piece of flint, or a burning string that the shooter placed in the flash hole by hand, or it may be a modern percussion cap that ignites the charge. These guns are still called by the method of ignition used: Matchlock, Flintlock, Cap and Ball, and so on.
Break Actions. This is most commonly seen in shotguns, though there are pistols and revolvers that use it. Here, the barrel is hinged on the front of the frame. When the action is opened, the barrel drops down and the chamber is exposed. Normally, this type of action uses a single barrel, but shotguns will commonly use two barrels mounted side by side, or one above the other. Some rifles will also have a double barrel configuration, though these present certain challenges. See Double Rifles.
Falling Block and Rolling Block actions were common in the US in the mid to late 1800’s. These tend to use big rounds and can only fire one at a time. The barrels are fixed in place, and the action is moved to load and unload them. Rolling blocks used a round metal breech that would rotate back to expose the chamber. Falling blocks used a lever near the trigger to drop the breech block down. In both cases, the weapon had to be cocked by hand.
Bolt Actions are the most common form of rifles in use today for hunting. They can be made in large calibers and are capable of extremely precision performance. Some are limited to one shot, while most hunting rifles have a five round internal magazine. To operate a bolt action, the shooter lifts up on the handle above the trigger and draws it back. Lifting up the handle cocks the firing mechanism, so the weapon is ready to fire as soon as the next round is loaded and the bolt is locked down. Very few have a way to de-cock them without just pulling the trigger. These are almost exclusively rifles, but there are some handgun and shotgun examples.
Pump Actions are very common on shotguns, and the sound they make is sure to get the attention of anyone that hears it. The spent shell is ejected, the weapon is cocked and a new round is loaded all in the time it takes to pull the fore end back and push it forward again. Additional rounds are normally stored in a tube magazine that runs under the barrel.
Lever Actions are similar to pumps, but use a lever under the trigger hand to eject, cock and reload the weapon. This is the classic 30-30 Winchester seen in every western ever made.
A Revolver uses a single barrel and a set of chambers that rotate, or “revolve” into position behind the barrel. When the round is fired, the bullet has to jump a small gap between the chamber and the barrel.
There are four types of revolver.
Single Action (SA) revolvers require that the hammer be pulled back by hand for every shot in order to cock them. If the hammer is down they cannot be made to go off on purpose.
Double Action (DA) revolvers can be cocked by hand, or they can be fired with the hammer down using the act of pulling the trigger to cock them. In the case of a double action, the hammer doesn’t quite get back all the way before falling forward and firing the gun. It’s not possible to use the trigger to cock the gun and leave it cocked.
Double Action Only (DAO) revolvers are just like the above, but they don’t have the option of cocking manually. These often have the hammers concealed and inaccessible.
In all three of the above, the act of cocking the gun, be it by hand or by pulling the trigger, causes the cylinder to rotate and bring the next round into position. How this is abused in writing will be detailed in the revolver pages.
The final type is the Automatic Revolver. No abbreviation for this since it’s so uncommonly used that people often assume the term is a mistake. It was made by Webley of England and would move the barrel and cylinder on the frame to force the hammer to cock and index the cylinder to align the next round. It did this by having a groove cut into the outside of the cylinder.
A Semi-Automatic (Semi Auto) is what most people mean when the say “automatic”. I’ll even do it here, so be ready. Semi-automatic means that the act of firing a single round will make the weapon ready to fire the next one. All ejecting, cocking and reloading has been done before the shooter is even ready to fire a second time. The difference between this and true automatic (or Fully Automatic) is that on full-auto, the second round will fire, then the third, fourth, and so on for as long as the trigger is pulled.
Semi-auto is an action that can be applied to pistols, rifles, and shotguns. Also, just because a rifle is semi-automatic does not mean it has a way to reload quickly via box magazine. Some have internal tube magazines, just like lever action and pump action rifles.
Another thing about semi-autos that can throw a lot of people is that they have similar sub-variations, like revolvers. The classic .45 Auto (1911) is a Single Action-Semi Automatic. There is no way to make it go off unless it’s cocked.
The M-9 Beretta, currently used by the US military is a Double Action Semi Automatic. It can be fired with the hammer down by just pulling the trigger. After it fires the first round, the hammer will stay back after each shot, making it a single action.
Some pistols, like the Glock and H&K VP70z have no external hammers. They may be DA or they may be DAO.