Certain weapons (higher level ones) have a synnergy. For example, I have a shotgun that has a synergy called run-n-gun. Based on how many mods of the same synergy I put on the shotgun, the stats increase. For example, the run-n-gun gives:
1) +5% movement accuracy 2) +10% movement accuracy 3) +10% accuracy and -10% recoil at low health 4) +20% accuracy and -20% recoil at low health.
If I only have 1 run-n-gun mod in it, I only get the extra +5% movement speed accuracy. However, if I have 4 run-n-gun mods in it I get all the bonuses.
"Immigrants. That's all they do, you know. Just driving around, listening to raps, shooting all the jobs." - Malory Archer GT: Bessy67