I need your help.Dao or da/sa ?

[Bowtie]

Looking to get a USP .45 But dont understand the DAO or Da/SA. I.ve only owned Glocks.I have a 30 , 17 ,19.Realy want a H&K.will be trading a new 17 in on one.Any help would be great.

[HAMMERHEAD]

DAO means Double Action Only, the trigger fully cocks the hammer (long, heavy trigger pull) for each shot like a revolver.
DA/SA means the first shot is double action (long heavy trigger pull), but the hammer remains cocked for the following SA shots (Single Action-short, light trigger pull).
H&K USP's can also remain cocked after loading with the safety on (cocked and locked).
Here's some good reading:
http://www.hkpro.com/