correct me if I am wrong but aren't most classic "western style" revolvers double action, meaning cock the hammer and squeeze?
that is single action, original revolvers' from the western days were single action meaning you have to pull the hammer back first before firing.
when you have to cock the hammer and then squeeze the trigger, it is single action. double action means you can do that or you can squeeze the trigger which will draw the hammer back and fire.
here you go, a good description:
SINGLE ACTION
We’ll begin with Single Action because this is where firearms got their start historically speaking. Using the technical definition of Single Action, black powder rifles and muskets that predate revolvers were single-action where the shooter had to manually pull the hammer back before pulling the trigger. So it’s easy to understand that in regards to revolvers, single-action means that one trigger pull equals one fired round, and that’s it. The shooter then has to cock the hammer to be able to fire the next round.
If you’ve seen any old western films that depict showdowns at high noon then you’ll recognize these revolvers as usually being single-action where the shooter had to pull the hammer back before firing each round. The benefit of a single-action firearm is that the trigger pull is both light and short. This allows you to squeeze the round off without having to exert much force through your hand which could affect your accuracy. Due to this benefit, many people find single-action triggers to be easier to shoot and a smoother experience. The downside is that if you are using a single-action revolver you will need to cock the hammer before every single shot.
Single Action semi-automatics are still the same as far as 1 trigger pull = 1 fired shot and needing to have the hammer manually pulled back before the first shot, but they differ from single-action revolvers in that the recoil from the fired round will cock the hammer for follow-up shots. This allows the shooter to remove the step of having to manually pull the hammer back before each shot which allows for subsequent shots to be put down range quicker than with a single action revolver. Due to the light trigger pull of single action, many people who have trouble with the heavier pull of double actions find these firearms much easier to shoot because the trigger pulls don’t require them to exert their hands as much.
DOUBLE ACTION
With double-action triggers, the difference between revolvers and semi-automatics become more apparent because of how follow up shots are handled.
A double-action revolver will have a trigger that both cocks the hammer and releases it in one pull and this will occur for every shot unless the hammer is pulled back manually before the shot. If the double action is used every time with the revolver, then the trigger will be uniform and the same pull length and weight with every shot. This allows the shooter to grow accustomed to the trigger quicker because it’s going to behave the same way every time it is pulled.
However, with most double-action semi-automatics, they are built to be DA/SA which is Double-Action to Single-Action. This means that the first round fired will be a double-action trigger pull, where the trigger both cocks the hammer and releases it in one pull, and then every follow-up shot will be single action. The way this works is that the firing of the round causes recoil which pushes the slide back, ejects the spent casing and chambers a new round. At the same time though, this recoil is resetting the hammer and arming it for the next shot, so the shooter does not have to cock the hammer manually or revert back to double-action trigger pulls again. This allows all shots from the 2nd onward to be shorter and lighter single-action pulls.
There are some firearms that are DAO (Double Action Only) but you will not run across them all too often in the semi-automatic world. Just understand that any semi-auto that states it is DAO means that every trigger pull will be double action. That is, heavy and long because it is doing both actions as described above.