MP9A2 Sub-Machine Gun makes quick work of lightly armored units
What the MP9A2 Sub-Machine Gun lacks in firepower it makes up for in speed. The weapon fires the fastest of any "common" weapon. The SMG is typically used by lighter classes, and against groups of light armor enemies. Often this will mean Gargoyle swarms. The SMG has the dubious honor of being the weapon to perform worst against foes with even a few points of armor, losing a full point of damage per point of enemy armor. The weapon is considered very inefficient against enemies with more than a point or two of armor, typically, anything larger than a Stalker.
The MP9A2 Sub-Machine Gun performs very well against enemies with debuffed armor, ie, armor that has been reduced by skills. This makes the weapon a good choice for Surveillance Recons. The SMG has no negative damage modifiers against massive or armored units, the naturally high armor of these enemies is what keeps the SMG ineffective against them. However, A "Surv Recon" with the Heartbeat Sensor is a passive +1 damage for his SMG. With proper use of Level 3 Laser Designator, he can add as much as 6 damage from reduced armor alone.
Other armor debuffs include the Field Engineer's Plasma Discharge and Field Probe's "Penetrate" skill. Bear in mind if the enemy still has at least 9 armor even after armor reduction, the SMG will still have minimal effect. This means the MP9A2 Sub-Machine Gun performs poorly against Eos the Zombie Queen, Slashers, and Stranglers unless there are several classes all using armor-reducing abilities together. The Rifleman can also increase the efficiency of the SMG with Focus Fire. If possible, it is a good idea to use Armor Piercing ammo with the SMG.