Like Tony said (and he's the true expert), ballistically the ShVak and the B20 are identical. In other words bullet drop is EXACTLY the same.
If your encountering something different, well... either your imagining it, or you were smoking something pretty serious last night.
If I remember correctly the overall armamemnt weight of the 3 cannon B-20 + Ammo, in comparison to the 2 cannon ShVak + Ammo, is within 10 pounds of each other.
In other words, fly the 3 cannon version. You get 30% more firepower, without any weight penalty.
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Vermillion
**MOL**, Men of Leisure