Sorry to make this question it may sound like I'm noob but that is what I am.
So, if i have a regular minifig (with 1d6 of skill) in size 3 vehicle with a size 2 a gun, this needs 3 to be used but it's size 2 so it's 6 skill to be used, right?
But if that gun was size 3 it would need 9 sklill to be used!
But my question is, if that gun was size 3 it would be almost imposible to my minifig to operate it, so a size 3 or more weapon in a vehicle it's useless?
no. for every 2" of size an object has, you get +1 to skill. so targeting a size 6 object with a size 3 gun would be (9 - 3) = 6 UR. keep in mind, that rolling a critical success means throw in a bonus die.