Totally missed this one.
Yes, that can be as (like Alanschu also noted) these weapons (like the assault gun) just do minor damage per hit. The real threat is from the amount of these bullets that can be shot.
Just compare headshotkilling with a pistol and a assaultgun. Pistol needs 1 round (easily managed with lvl 2 pistols), assault gun needs 5 (all hit, which usually requires about lvl 3 Rifles and mods)
No damage from the tranqs? How did you even be able to check that anyway, since the first burst of damage is VERY shortly after impact?