How do you know how much damage you do per gun?
Erm, well, i guess you just use Guns until you get the achievement. Done.
Then on the next playthrough you use another combat skill.
After 5 playthrougs you will have all the damage dealing ones.
Or you could always combat spec.
So that once you get the achievement for Guns you start using EW only until you get the next, then start using Unarmed until you get the next and continue on and grind and explore.
Cake-walk achievements.