Short, easy question for most of you.
 

If a semi-auto pistol is single action, that means that it has to be cocked only once per magazine (on the first round), correct? Once the first round is shot, it will re-cock itself until the mag is empty?

Single action refers to the number of things that happenwhen you pull the trigger. In the case of a ingle action, it simply drops the hammer. In a double action you cock the hammer and drop it when you pull the trigger.

Most pistols that are SA are also DA, meaning you can fire it with the hammer back or forward with just a pull of the trigger. The notable exception is the vaunted 1911, which is SA only.

Yes. As long as you understand that it is still possible to manually drop the hammer before you finish the magazine.

A Single Action semi-auto will then have to be manually cocked for it to be ready to fire again. Then the next trigger press will fire.

A Double Action semi-auto doesn't require that you cock it manually. If the hammer is down on a loaded chamber a trigger pull can both cock and release the hammer to fire the next round. Note that most DA semi-autos are actually DA/SA in that aside from firing with the hammer-down on a loaded chamber (aka. "first shot") the remaining shots are SA ... ie. indistinguishable from a SA pistol.

Then there are the "DAO" Double Action Only and "Glock Action" types of pistols ... but I won't get into them now for the sake of brevity.

That's true of ANY semi-auto pistol regardless of it being SA or DA.

That's all I wanted to ensure. I was just double-checking. Now that I've got it pat, I know what the Glock "safe action", etc are as well.

Let it also be known that a single action gun has a heavy, long trigger pull the first shot, however the rest of the shots have a very light and short trigger pull. Some may find this easier to shoot straight. (Actually I am not sure that all SA guns have the heavy trigger pull for the first shot, maybe someone could verify this)

A double action gun, on the other hand, has heavy and long trigger pulls for all shots.

I hope this makes sense to you, and gives a reason for the two kinds of actions.

Painted, you're incorrect.

First of all, the question was specifically about SA vs. DA semi-auto pistols, not revolvers.

To be clear:

SA semi-auto (e.g. Colt 1911, Browning HP) - trigger pull is always relatively short/light. It's just that the hammer needs to be cocked for the first shot. When you rack the slide the hammer is cocked so you're ready to go. But if the hammer is "down" you can't even pull the trigger or fire - it has to be manually cocked. (Usually a SA auto is holstered with the hammer back and safety engaged - this is what's known as "cocked and locked") All trigger presses are relatively short/light.

DA/SA semi-auto (e.g. Sig Sauer P229, HK USP) - exactly the same as the SA semi-auto except that if the hammer is "down" you don't have to manually cock the hammer. In this case a single trigger press (DA) will cock the hammer then fire the gun. This is the relatively long/heavy trigger pull you were thinking about. If you don't like the heavy trigger pull of the DA you can still manually cock the hammer for the first shot.

SA revolver (e.g. Colt Peacemaker, Ruger Blackhawk) - you have to manually cock the hammer for each shot. If the hammer is down you can't even fire.

DA revolver (e.g. S&W M19, Colt Python) - Again, you don't have to manually cock the hammer to shoot. You can press the long/heavy DA trigger for the next shot.

I did not reference revolvers in any way. I don't know where you got that.

Although I respect your knowledge of pistols (as it is far greater than mine), I remember shooting Pops' pistols, and I remember that the guns in double-action always had long trigger pulls, and the guns in SA had light trigger pulls. Maybe I remembered incorrectly. I do know that one of his guns was a DA hammerless, and it was a tiny thing, difficult to shoot.

Painted, I thought you might have been referring to revolvers because the only item that has a "heavy long trigger pull for all shots" is a DA revolver.

The "hammerless" ones you mention might have been Glocks or one of it's equivalents (Walther 99, springfield XD, S&W M&P). I own and shoot many types of firearms including Glocks. Again, it's trigger pull isn't long but it is slightly heavier. It is neither DA or SA. Technically it might be called a "1.5 action."

As I mention above, a true DA semiauto will have a long heavy trigger pull ONLY with the first shot. The whole point of making a semiauto is that each shot recocks the hammer for a lighter SA trigger pull.

But if you fired a semiauto which had a long heavy trigger pull with each shot, then it's not because it's a DA auto, it's because the gun needs a good trigger-job. Ask your dad.

Double action only = Long heavy trigger pull every shot
Double action = Long heavy pull on the first shot for autos, and long pull on every shot for revolvers unless the hammer is cocked manually. Anyone bringing up the Mateba gets a swift kick in the nuts.
Single action = Light trigger pull for all shots, on an auto the hammer must be cocked manually by hand or by racking the slide, on a revolver it must be cocked by hand.

Glock, Certain Walther P99 variants, Springfield XD, HK LEM etc. are all striker fired so the terminology get's a bit muddy. Each gun precocks the striker to varying degrees, on a, on the Glock is around say 30-40% cocked, on an XD it is around 95% cocked. All are classified as DAO although the XD is really SA and can have a trigger pull below three pounds.

Certain non-striker fired autos that are classified as DAO (Para LDA comes to mind) have a lighter DA pull by precompressing the mainspring after each shot.

The precocking/precompressing done by SA XDs, Glocks, and Para LDAs is the reason they cannot be dry fired more then once without racking the slide. The P99 has a superior mechanism on all variants that allows for double strikes. Sig has a new system called DAK that offers a lighter DAO pull by offering increased leverage.

You won't have to cock a single action autoloader ever unless you chose to manually drop the hammer (not usually a good idea). When you load a round in the chamber by cycling the slide the hammer is cocked automatically and will be recocked every time you shoot and the action is cycled.

I had to cock the hammer manually on my 1911 just the other day. It was a hard primer or the grip safety failing to fully enage the firing pin safety. It happens.