A firearm includes a frame having at least one interior receptacle defined by opposed interior walls and having opposed guide rails extending from the opposed interior walls, and at least one modular housing disposed in the receptacle and accommodating at least one operational component of the firearm. The modular housing is defined by opposed sidewalls having opposed guide channels formed therein, the guide channels being configured to selectively engage the guide rails to facilitate the aligning of the modular housing with respect to the interior receptacle. The at least one operational component and the modular housing are pre-assembled together as an integrated unit for drop-in insertion into the receptacle during assembly of the firearm.