Machine Guns have appeared in every SFH game so far. It is a good spray-and-pray weapons, being able to combat several enemies while using massive amount of bullets before needing to reload. They should be used aggressively to exercise their potential.
Strike Force Heroes
Machine Guns are primary weapons for the Commando. They are similar to the medic's Assault Rifles, but machine guns have usually a better fire rate than the assault rifles and almost all machine guns have the highest ammo in the game, with an exception of OICW. Their downside is the low accuracy and range. You can suppress enemies easily at close range by firing many bullets to the enemy without having to reload, meaning that machine guns are the easiest way to kill Tanks.
Strike Force Heroes 2
Replacing the Commando, the Mercenary is now the sole user of this class of weapon. It should be used in much the same way.
Strike Force Heroes 3
The Mercenary now shares this weapon with the Medic. Its damage per shot has dropped in comparison to the previous games, therefore, it feels much weaker.