To get the MG42 light machine gun in Far Cry 6, you have to complete the Sweet Fifteen treasure hunt, for which you are also rewarded with the El Unicornio horse. The location of the MG42 is the Torres Warehouse, north of Savannah Fields in Sierra Perdida, El Este.
Go to the Torres Warehouse and begin Sweet Fifteen by reading the note by the office door on the north side of the warehouse. Now go inside and find your way into the main room, where there’s a control panel with various wires leading out of it. To complete the treasure hunt, you have to follow the wires and throw four power switches, before returning to the control panel and hitting the big green button. Find a complete guide on how to do that right here.
When you hit the big green button, the Sweet Fifteen party will begin, and the door to the south will open. The room beyond is the location of El Unicornio and the MG42, both of which will be rewarded for completing the treasure hunt. It’s an excellent weapon even in its basic form but, when fully modded out, the MG42 is the best light machine gun in Far Cry 6.