I posted this in another thread, but it was suggested that it warrants its own thread, so here goes:

Ok, I haven't really found a clear-cut answer to my question, so I'll piggy-back it onto this thread (I received a partial answer in another thread but didn't want to derail from the original topic too far). What is the difference between SAO, DAO, DA/SA, and SA/DA triggers in a semi-auto pistol? I'm going to be new to semi autos in the near future, and am just looking to get more informed about the multitude of options available.

I understand the difference between SA and DA when discussing revolvers, but when the action becomes combined on a semi it confuses me a bit. It was my understanding that when the slide moves backward after firing, it ejects the spent shell and re-cocks the hammer, and then when it moves forward it chambers the next round so it is ready to fire again.

This is why, I thought, the de-cocker (sp?) that some models have, would bring the hammer to a rest and render the pistol not live until it was re-cocked. It was always my impression that semi autos were single action triggers, and the slide simply re-cocked the hammer, providing the second action.

If it is a DA trigger, then the slide re-cocking the hammer really isn't necessary is it?