DA only, racking it only chambers the round and the trigger does everything.
Basically yes... with a couple of caveats.
Many DAO pistols use a pre-set hammer or striker
that is initially partially cocked by the slide. When the trigger is pulled, the hammer or striker is drawn back the rest of the way and released; the hammer or striker is then partially cocked again as the slide cycles. This system reduces the length and/or weight of the DAO trigger pull.
The reason this is significant is that many pre-set DAO pistols lack second-strike
or repeat-strike capability
- the ability to pull the trigger repeatedly and hit the same round multiple times if it does not fire on the first attempt. Pistols that lack this capability MUST be manually cleared in the event of a failure-to-fire, because the trigger does nothing once the hammer or striker is decocked. Common pistols that lack this capability include Glocks, the S&W M&P and Sigma series, Kel-Tecs, and the Ruger LCP and LC9.
Some pre-set DAO pistols do
have second-strike capability, although a subsequent trigger stroke is usually longer and/or heavier than the initial DA pull; examples include HKs with the LEM trigger and SIGs with the DAK trigger.
Other DAO pistols do not have pre-set hammers or strikers, in which case the hammer or striker always starts from the same position, giving the pistol second-strike capability by default. Common examples include the Beretta 92D and 96D and the SIG P250. (In other words, the trigger works basically the same way as the trigger of a DA revolver, minus the ability to hand-cock the hammer for single-action fire.)
Other related notes...
Almost all traditional DA/SA hammer-fired pistols have second-strike capability.
A handful of DA/SA hammer-fired pistols feature a half-cock notch
. Half-cock notches were originally intended as a safety feature on older SAO designs (although they were often dubiously effective as such); however, on a DA/SA pistol, they generally serve the same purpose as a pre-set hammer on a DAO pistol- i.e. reduction of the DA pull length and/or weight. DA/SA pistols with this feature include the CZ 75 and early versions of the Beretta 81-series (which includes the 84 through 87).