Hi there. A looooong time ago, i have read a review of a SCI-FI FPS Game where you had only one weapon and a lot of kinds of ammo. The weapon itself could fire rockets, bullets, and so on...

I remember that the game was pretty lame (enemy AI was poor, poor level design...) .

Did you remember a similar game? I can't remember the platform, maybe Windows 9x or MS-DOS

If you want a game with better ratings and very similar to that, look into "Oddworld: Stranger's Wrath" that gives you just one weapon but a dozen ammo types (For the first half of the game. Guns go away later in the game)

Codename Outbreak sounds about right.

I found it to be a rather cool and underrated game tbh, a bit like Delta Force or Ghost Recon with an alien invasion plot.

Same here. ;)