I haven't read all the 16 pages of comments, so it might be already suggested.
Tracer rounds: Embeds it self into the target and produces light for a short time. Helps to find targets at night, in fog or sandstorm.
Marks the enemy for 30 sec on hit (like the spyglass).
-20% clip size or -20% muzle velocity.
Dirty ammo: Ammo filled with all the nasty stuff (tar, glue, etc.). On impact it shatters and covers the components of the machinery, making maintenance a nightmare. How ever the ammo is prone to leaks, so it makes the gun's maintanace just as miserable.
After hit it applies a debuff that reduces repair efficiency by 15% on modules struck for 5 second. Further hits just refresh the debuff, they do not stack.
-15% clip size, -50% gun rotation speed + the same debuff on the gun after the shot.
Shredder rounds: Ammo designed to properly smash appart and shred modules. The rounds are larger and heavier then normal.
When a shot destroys a module it applies a debuff that lasts indefinetly, reducing rebuild power by 25% on that module. (Spanner: 5-->4, Wrench: 4-->3, Mallet: uneffected)
-20% clip size, -25% muzle velocity
Short fused rounds: For guns that use ammo that has arming time. Fuse set shorter, for close(r) range combat.
-50% arming time for shots that have it (not by reducing muzle velocity).
-20% rate of fire, -10% damage, -15% gun rotation speed