Sucking teeth, is more done with the lips. Usually it's a sign to show displeasure with something. For example, "*Sucking teeth noise* Man, I really thought I had a good deal". You've have to have heard the noise at some point and probably do it yourself. The name "sucking teeth" threw me for a whille too, when I didn't know there's actually a name for the noise.
Cracking gun, is more like snapping. Some people are better then others, but if someone's decent, it can be a very loud, snap or cracking noise, and can be repeated very frequently.
