There is no gun skill ("Power Shot"?), per se - guns, like crossbows, have only a weapon proficiency.
The only skill that might help you fire a gun is Horse Archery, but that's only if you're riding a horse. Think of it not as a restriction ("Aww, my guns can't be more overpowered than they already are") but as an opportunity ("This means I can pump all my points into Charisma and Leadership so I can have more guns!!").
Of course, despite using guns you'll still be at risk from enemy fire, so you'll still want to put some points into iron flesh or at least strength to use some decent armor, etc.