So it would come out to (2d(4+1+1))x2, 4-24, avg 12 damage? Assuming both shots hit and I have no other traits.
No, shark has the formula correct - the trait bonuses are applied to the result of the dice roll, not to the dice sides. SoG and SoB mean their respective levels, so with both of them maxed up to 5 it's (2d4+10)x2 = 24-36, avg. 30, assuming both shots hit.
Other traits are IIRC irrelevant anyway, except for MSs, which always deals maximum possible damage, but blocks SoB and dual-wielding.