An injector apparatus, comprising: an elongate outer casing having a distal end and a proximal end; a plunger movement assembly, comprising: (a) a screw axially disposed within the outer casing; the screw having helical threads; a nut wherein the nut has a pin or pins that ride in the threads of the screw; wherein the screw has external threads and the nut is disposed around the screw; wherein the screw having helical threads comprises threads in a first portion that turn in a first direction, and that turn in a second direction in a second portion; and wherein the nut has a pin or pins that ride in the threads of the screw such that the nut turns in the first direction in the first portion and in the opposite direction in the second portion; a combination compression and torsion spring (22, 50) that is connected at the distal end to the casing and connected at the proximal end of the spring to the nut; a plunger rod connected to the proximal end of the nut; or (b) a nut comprising an axial central cylindrical orifice having helical grooves; a screw flange disposed within the central cylindrical orifice having a pin or pins that ride in the helical grooves; a plunger rod connected to the screw flange; and a combination compression and torsion spring that is connected at the distal end to the casing and connected at the proximal end to the screw flange; wherein the nut having helical grooves comprises grooves in a first portion that turn in a first direction, and that turn in a second direction in a second portion; and wherein the screw flange has a pin or pins that ride in the grooves of the nut such that the screw flange turns in the first direction in the first portion and in the opposite direction in the second portion; a syringe adapted for containing a medicament attached to the outer casing and/or a proximal end of the PMA; and wherein the proximal end of the plunger rod is slide-ably disposed within the syringe. an elongate outer casing having a distal end and a proximal end; a plunger movement assembly, comprising: (a) a screw axially disposed within the outer casing; the screw having helical threads; a nut wherein the nut has a pin or pins that ride in the threads of the screw; wherein the screw has external threads and the nut is disposed around the screw; (a) a screw axially disposed within the outer casing; the screw having helical threads; a nut wherein the nut has a pin or pins that ride in the threads of the screw; wherein the screw has external threads and the nut is disposed around the screw; the screw having helical threads; a nut wherein the nut has a pin or pins that ride in the threads of the screw; wherein the screw has external threads and the nut is disposed around the screw; wherein the screw having helical threads comprises threads in a first portion that turn in a first direction, and that turn in a second direction in a second portion; and wherein the nut has a pin or pins that ride in the threads of the screw such that the nut turns in the first direction in the first portion and in the opposite direction in the second portion; a combination compression and torsion spring (22, 50) that is connected at the distal end to the casing and connected at the proximal end of the spring to the nut; a plunger rod connected to the proximal end of the nut; or (b) a nut comprising an axial central cylindrical orifice having helical grooves; a screw flange disposed within the central cylindrical orifice having a pin or pins that ride in the helical grooves; a plunger rod connected to the screw flange; and a combination compression and torsion spring that is connected at the distal end to the casing and connected at the proximal end to the screw flange; a combination compression and torsion spring (22, 50) that is connected at the distal end to the casing and connected at the proximal end of the spring to the nut; a plunger rod connected to the proximal end of the nut; or (b) a nut comprising an axial central cylindrical orifice having helical grooves; a screw flange disposed within the central cylindrical orifice having a pin or pins that ride in the helical grooves; a plunger rod connected to the screw flange; and a combination compression and torsion spring that is connected at the distal end to the casing and connected at the proximal end to the screw flange; (b) a nut comprising an axial central cylindrical orifice having helical grooves; a screw flange disposed within the central cylindrical orifice having a pin or pins that ride in the helical grooves; a plunger rod connected to the screw flange; and a combination compression and torsion spring that is connected at the distal end to the casing and connected at the proximal end to the screw flange; wherein the nut having helical grooves comprises grooves in a first portion that turn in a first direction, and that turn in a second direction in a second portion; and wherein the screw flange has a pin or pins that ride in the grooves of the nut such that the screw flange turns in the first direction in the first portion and in the opposite direction in the second portion; a syringe adapted for containing a medicament attached to the outer casing and/or a proximal end of the PMA; and wherein the proximal end of the plunger rod is slide-ably disposed within the syringe.