Incendiary Ammunition:

This ammunition deals 100% of the damage on your gun, plus an additional 10 damage. Further shots reset the duration of the extra damage but does not add additional damage. Incendiary protection prevents 20% of the base damage from applying before the DoT.

Toxic Ammunition:

This ammunition does 100% of the damage and prevents healing past 100 health. Additional shots do not affect damage, but they do add the the end damage over time. This is why one shot from a toxic SMG seems to do less damage than multiple shots from the same SMG. Toxic Protection prevents 20% of the damage and removes the healing prevention.

It seems that Incendiary is the best ammo for a high damage setup, while toxic can assist with teammates killing enemies. Normal ammo prefferable only for its consistency. The damage number on your weapon will never change if you use normal ammo.

Examples of damages on a 50 damage Assault Rifle:
Normal- 50 (No Bullet Protection)
Toxic- 50 (40)
Incendiary- 60 (50)

Two shots:
Normal- 100
Toxic- 100 (80)
Incendiary 110 (90)

Incendiary slowly loses its effectiveness over prolonged shooting, while toxic merely prevents depot healing and regen over 100 hp. The use of Incendiary Protection is more effective the longer the opponent fires, while toxic protection simply reduces the total damage.