Most machine guns have extremely high rates of fire. It is their primary strength. SAW and Chain SAW have especially high RoF. However, machine guns have very bad accuracy. For most of weapons of this type, accuracy is the worst in the game. Because of this, Machine guns are best at close range. They have longer range than Elemental Weapons, but due to the bad accuracy most of the shots you fire probably won't hit the enemy.
Default machine gun for both Medics and Mercenaries is MK48.
Use Pistols and SMGs as secondary weapons to make long range combat easier.