Edit the damage=x setting in the weapon's gunspec.txt.
Gunspecs can be found in Game Guru > Files > Gamecore > Guns
If you already have a weapon placed in the map it may not accept changes and will have to be replaced. Be sure to close and relaunch GG after editing any gun settings.
Alternatively, you could just increase the player hitpoints from the player start marker properties.
Gigabyte P67A-UD4-B3, Intel Core i7 2600K, 16GB Corsair DDR3, EVGA GTX 970 SC, Win7 Pro 64-bit SP1, Primary monitor @ 1920x1080, secondary monitor @ 1024x1280
