- Combat shotgun down to mil 2 (from 3)
- SMG up to mil 2 (from 1)
- Piercing ammo (both types) to own tech with red/green/grey sci & mil 2 required
Shotgun over SMG has 2 advantages:
- More efficient on ammo, but that's not a concern when actively in combat
- Faster at killing spawners... if you're right next to it, with no biters left
By the time combat shotgun is unlocked at mil 3 (requires blue sci) far more effective options are available - rockets, capsules, tank, SMG & piercing ammo. It is an early game weapon that isn't unlocked until mid game.