A single-action (SA) gun has only single-action capabilities.
A double-action (DA) gun has single-action and double-action capabilities.
A double-action only (DAO) gun has only double-action capabilities.
SA, DA, and DAO may be applied to revolvers or semi-autos.
SA/DA only applies to semi-autos.
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SA (Single-action) = The gun's hammer/striker must first be fully cocked manually or by cycling the slide, the trigger's only job is to release the hammer/striker.
DAO (Double-action Only) = The gun may have one or more of these capabilities only:
The hammer/striker may be fully cocked by the trigger, and released by the trigger.
Or the hammer/striker may be half-cocked by cycling the slide, then fully cocked and released by the trigger.
DA (Double-action) = The gun must have one or more of these double-action capabilities:
The hammer/striker may be fully cocked by the trigger, and released by the trigger.
Or the hammer/striker may be half-cocked by cycling the slide, then fully cocked and released by the trigger.
AND the same gun must have one or more of these single-action capabilities:
The hammer/striker may be fully cocked manually or by cycling the slide, then released by the trigger.
Or the hammer/striker may be half-cocked manually or by cycling the slide, then fully cocked and released by the trigger.
SA/DA (Single-action/Double-action) = The gun must have both of these capabilities:
If the slide has been cycled it fully cocks the hammer/striker, and the trigger's only job is to release the hammer/striker (SA).
Or if the slide has not been cycled the trigger will fully cock the hammer/striker and then release the hammer/striker (DA).