When i check the damage stats on damaging bullets it says 2-4, in my head it would means there is a random damage chance of dealing between 2 - 4 extra damage per shot, but it is not the case in the game, so what does those 2 numbers means? 2 for small weapons (SMG) and 4 for heavy (snipers) ?

Hey, good question. In DOE, damage doesn't randomize, but we do have critical hits. Each weapon has a critical percent stat that is used to randomly calculate when a critical hit occurs. It is much higher on a flanked character. Also, some equipment and skills can influence critical hit percentage. 

So you probably guessed by now, but the first number is standard damage and the second is critical hit damage.

I'll update the hover description of attack to make that more clear! Glad you asked!