Extended Magazine

Fit it to the gun that keeps running dry in the middle of a fight. Most deaths in a base defense happen during a reload: you drop the first raider in the corridor, the second one rounds the corner, and you are watching your hands push a new mag in. A bigger magazine moves that moment further back, sometimes past the end of the fight.

It costs ten high quality metal at a Level 2 workbench, the same as the other mid-tier mods, and it takes the magazine slot, so it does not fight your sight or laser for space. On a spray gun like the MP5A4 it buys a few more seconds of fire in a doorway. On a semi-auto like the M39 it means more shots at a roamer before you have to duck behind a rock.

Research it early. One blueprint covers every gun that accepts it, and moving the magazine from one rifle to the next takes a second in your inventory.