Gunners don't make sense

Having just fired a gun for the first time the other day, I can say that it definitely requires a great deal of strength for maximum effectiveness.

But you have to remember in RPG-land even the waifiest of girls has more strength than the average well built real life man. ;)

Well, guns are more powerful than any melee weapon no matter who is pulling the trigger. Adrenaline has the capability to induce immeasurable strength even in the daintiest musculature so upper body definition is really only necessary when factoring endurance and successive firing.