Machine guns are fully-automatic firearms designated for extended periods of firing. They often have very large ammunition capacities.
Technically speaking, most of the Machine guns in Resident Evil are in reality Submachine guns (SMG), rapid-fire automatic carbines chambered for handgun cartridges. True machine guns are massive-sized firearns chambered for rifle cartridges, usually fed by ammunition belts, and have slower rate of fire compared to SMG's.
For the sake of convience however, both SMG's & Machine guns in the Resident Evil series are classified as Machine guns. They are not to be confused with Assault rifles. ( A notable execption is in Resident Evil 5, where both Assault Rifles and Submachine guns are classed as Machine guns )