Accuracy accumulation?

[Nanaki]

eg. if u chose laser proficiency + 40% gunner, how accurate when u r using laser?
30 + 20 +40? or
1.3 x 1.2 x 1.4? or
1.3 x 1.4?

You start off with an 20% accuracy penalty, or 0.8. It than adds all the bonuses, so .8 + .3 + .2 +.4, which equals 1.7, or +70% accuracy. You can see all the calculations in the Data/Script/AI/PerkDispatcher.script file.

and what does a low/mid/high accuracy weapon mean exactly?

Most weapons have a certain 'degree' of scatter, the higher the accuracy of the weapon, the lower the scatter.