I'm very not familiar with the HK USP line, though after shooting a USP9 compact, I am seriously considering purchasing one. However, my knowledge of the USP line (or technical firearm knowledge for that matter), is relatively limited. I've seen the USP variant charts, however, I am still unclear as to what some of the fire modes and control functions do individually, as well as in combo with others. Can someone please educate me so I can make the correct variant choice? Thanks in advance.