A module housing is adapted to be inserted into an operating position in the trigger group receiving area of a firearm. The module housing includes at least one pin receiver defined by two openings formed in the module housing, one on each lateral side of the housing. A module pin is received in the pin receiver and at least one trigger group component is mounted or supported for rotation on the module pin. The pin receiver is located on the module housing so as to align with a pin receptacle of the firearm when the module housing is in the operating position. By locating the pin receiver in the module housing so as to align with a corresponding pin receptacle of the firearm when the module housing is in the operating position, the trigger group module and the trigger group components housed in the module housing may be readily supported by the OEM pin receptacle. The trigger group module, pre-assembled with one or more trigger group components, may be inserted to the operating position and then held in place using the OEM pin receptacle.