I've done a little experimenting in split screen and online and I am starting to find the damage of each gun. The normal health of a target is 100, but bio booster makes it about 120-130 (not sure yet). This is the damage of some of the guns (all splitscreen guns except RPG) from what I see, High-hitter adds 5-10 damage. Shots aren't based on range, but randomness of each shot/where you hit except shot guns.
PT9 Interdictus (shotgun)
130-100 Point blank - 15 meters
80-40 20 meters-50 meters
Masterton M-557 (shotgun)
130-100 Point blank - 20 meters
80-40 25 meters-55 meters
With these shotguns past the last range you aren't going to hit or significantly damage an enemy.
Ivana Spec R
Hopefully I'll be able to get more guns by picking them up online