Glocks are Double Action Only. This has come up many times on other sites so I took the liberty of calling Glock within the past year to speak to a technical person. He assured me that yes, Glock was classed as a DAO pistol.
Glock can class the Glock as whatever Glock wants to class it as. But neither mechanically nor functionally speaking is the Glock trigger the same as the mechanism in a double action only revolver.
At once there was a time where the only handguns that mattered were revolvers. There were revolvers with single action triggers, which you had to cock each shot before the trigger would fire the gun. There were revolvers that were double action, meaning that the trigger would cock and fire the gun and you didn't have to cock the gun first before the trigger functioned.
At that time, double action and single action had clear meanings. Then people started using semi-auto pistols. You might say they were like a double action revolver in that each pull of the trigger will fire the gun without having to take a separate step. But the trigger didn't cock the hammer, the slide did. The trigger mechanism itself was actually very similar to a single action revolver, whereby the slide basically did the work of the finger that cocked the hammer. That's why later guns like the Beretta 92F were called double action. On the 92F, you could decock the hammer, and the trigger would by itself fully cock and fire the gun, but the slide also would cock it. In this case, the trigger worked almost in the same way as the double action revolver trigger, except that not only could you cock the hammer by pulling the trigger or manually using your fingers, but the slide could cock it just like with the "single-action" semi-autos.
On a double action only revolver (S&W 642 for instance), the trigger cannot be kept in a cocked condition or cocked by hand. The 92D worked in the same way, where you could neither cock the hammer by hand nor would the recoiling slide leave the hammer in a cocked state.
A Glock may be classed double action, but it does not work in the same way as these other mechanisms do. For one thing, a Glock doesn't have a hammer, having a striker instead. But even beyond that, its striker cannot be cocked by the trigger alone.
But it's all just a name. There is obviously a difference, even if they call them the same name. It has gotten to the point where the label "double action" is basically meaningless. It only really has meaning if you are talking about a revolver.
Heck, even a double action pistol like the 92F has a trigger mechanism that is different from a revolver. In a revolver, not only does the trigger cock the hammer, but the hammer also rotates the cylinder when it's cocked. So in a way, a revolver trigger mechanism does even more.
A Colt single action army is clearly single action. An S&W Model 10 is clearly double action. An S&W 642 is clearly double action only.
But is a 1911, M9, 92D, SIG DAK, H&K LEM, Springfield XD, Kahr or Glock double action or single action? Well, their manufacturers all claim they are something, but obviously they are not identical in operation.
Classing a Glock as Double Action has serious advantages, when the government is only accepting double action guns as candidates for their replacement guns.
Also, here is an interesting tidbit about how well a partially cocked striker in Glock works as a safety mechanism:
http://www.americancopmagazine.com/articles/xd/index.html <--
There is a degree of controversy over how much the striker spring is "pre-loaded." Critics say since the Glock's is only partially loaded it is safer than those that are fully compressed. Springfield commissioned a study by the prestigious independent consulting firm, Tioga Engineering of Wellsboro, Pa., to compare the systems of the Glock and XD. In both designs, the firing pin block or safety is the key element, but their test showed that a primed case fired 100 percent of the time in a Glock when the striker was released from the 62 percent normal preloaded level.