SAO is the Variant 9. Works like a standard single action with thumb safety ala 1911. The control lever looses its de-cock function.
I prefer it over Variant 1 since there is no DA/SA trigger pull transition to worry about. Also, and in my opinion more importantly, if you ride the safety ala 1911 when you shoot the USP Variant 1, if you grip too strong you may find yourself pushing down the de-cock lever slightly, which will not allow the pistol to fire. I found myself doing this a few times when shooting under stress. Not good if you needed the gun to go bang real world. Variant 9 eliminates this problem, as the control lever becomes just a safety with the de-cock function disabled.
As for the conversion to Variant 9, all you need is a new detent plate (costs $5-10 dollars or so) that you can order direct from HK customer service. They actual swap is easy as pie and takes about 5 seconds (maybe 30 seconds if it's your first time).
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