Compare to single action
and double action
firearm modes of fire.
Double action only or DAO is a mode of fire which resets the hammer to the lowered or uncocked position after every shot. In revolvers this is the default mode of operation.
In auto pistols the backward movement of the slide pushes the hammer back and cocks it to make subsequent shots single action, by modifying the action the hammer can be made to follow the slide down and back without firing the next round. This makes the auto pistol behave like a traditional revolver.
DAO guns are commonly issued to Police Agencies as a lot of them were trained using revolvers and having to retrain all of them again in the use of new equipment would just be a logistical nightmare. Also DAO guns have a heavier trigger pull than single action guns and are less likely for the operator to have a negligent discharge. As always the four rules of gun safety must be observed at all times.