For the weapon itself.
The main benefit of a gun is its ease of bypassing armor. Otherwise, there little difference between a sword thru the abdomen and a bullet thru the abdomen. Both occur when reaching zero hit points, where the zero means a potentially fatal wound.
One way to represent it is, the gun imposes a Reflex save, for full or no damage. Then agile light armor with high Dex is difficult to damage, while low-Dex heavy armor is an easier target.
Either way, the idea of moderate d8 damage being more if a crit, makes sense.