A perforating gun, comprising: a gun body having external threads at a first end and internal threads at a second end; a shaped charge loading tube assembly, received in the gun body and including a conductive charge tube, having an upper end and a lower end, an upper end fitting (218; 218A) on the upper end of the charge tube, the upper end fitting being conductive, and an upper insulating cap (210; 210A) on the upper end fitting (218; 218A) between the shaped charge loading tube assembly and the gun body, and a lower end fitting (240; 240A) on the lower end of the charge tube, the lower end fitting being conductive, and a lower insulating cap (230; 230A) on the lower end fitting (240; 240A) between the shaped charge loading tube assembly and the gun body; and a cartridge, including a cartridge body having an upper end and a lower end, a detonator proximate the upper end, a switch electrically connected to the detonator, a first electrical contact proximate the lower end, and a first electrical contact proximate the upper end, wherein the first electrical contacts (340, 379) are electrically connected to the switch, and wherein the lower end fitting (240; 240A) has an electrical contact adapted to contact the first electrical contact proximate the upper end of the cartridge; wherein the first electrical contact proximate the lower end of the cartridge is adapted to contact an electrical contact of an upper end fitting (218; 218A) of a shaped charge loading tube assembly of another perforating gun when the other perforating gun is assembled with the perforating gun. a gun body having external threads at a first end and internal threads at a second end; a shaped charge loading tube assembly, received in the gun body and including a conductive charge tube, having an upper end and a lower end, an upper end fitting (218; 218A) on the upper end of the charge tube, the upper end fitting being conductive, and an upper insulating cap (210; 210A) on the upper end fitting (218; 218A) between the shaped charge loading tube assembly and the gun body, and a lower end fitting (240; 240A) on the lower end of the charge tube, the lower end fitting being conductive, and a lower insulating cap (230; 230A) on the lower end fitting (240; 240A) between the shaped charge loading tube assembly and the gun body; and a conductive charge tube, having an upper end and a lower end, an upper end fitting (218; 218A) on the upper end of the charge tube, the upper end fitting being conductive, and an upper insulating cap (210; 210A) on the upper end fitting (218; 218A) between the shaped charge loading tube assembly and the gun body, and a lower end fitting (240; 240A) on the lower end of the charge tube, the lower end fitting being conductive, and a lower insulating cap (230; 230A) on the lower end fitting (240; 240A) between the shaped charge loading tube assembly and the gun body; and a cartridge, including a cartridge body having an upper end and a lower end, a detonator proximate the upper end, a switch electrically connected to the detonator, a first electrical contact proximate the lower end, and a first electrical contact proximate the upper end, wherein the first electrical contacts (340, 379) are electrically connected to the switch, and wherein the lower end fitting (240; 240A) has an electrical contact adapted to contact the first electrical contact proximate the upper end of the cartridge; a cartridge body having an upper end and a lower end, a detonator proximate the upper end, a switch electrically connected to the detonator, a first electrical contact proximate the lower end, and a first electrical contact proximate the upper end, wherein the first electrical contacts (340, 379) are electrically connected to the switch, and wherein the lower end fitting (240; 240A) has an electrical contact adapted to contact the first electrical contact proximate the upper end of the cartridge; wherein the first electrical contact proximate the lower end of the cartridge is adapted to contact an electrical contact of an upper end fitting (218; 218A) of a shaped charge loading tube assembly of another perforating gun when the other perforating gun is assembled with the perforating gun.