What is the difference between single action and double action semi-auto handguns?

Question

I think I understand these two terms as they apply to revolvers, but I've heard of semi-autos being classified as double or single action as well.

So what's the difference and is one type safer than the other?

Thanks.

Answer 1

Single Action: Pulling the trigger does ONE thing : if the hammer has been cocked, it will release the hammer. Trigger pull is light, short, and consistent between first and subsequent shots.

Double Action: Pulling the trigger does TWO things : it will first c**k the hammer if it hasn't been cocked, and then it will release the hammer. DA Trigger pull is longer and heavier than SA, but the user can c**k the hammer first if they want to fire in "Single Action Mode." If the gun is semi-automatic, the first shot is DA, subsequent shots are SA due to the slide cocking the hammer for you, resulting in differing trigger feel as described above.