An automatic injector adapted to receive a cartridge including a barrel , a needle , and a plunger assembly including a plunger seal , the cartridge defining a longitudinal axis (A), and the injector comprising: a housing ; a cartridge carrier adapted to receive at least a portion of the cartridge , the cartridge carrier being disposed for movement relative to the housing in a direction parallel to the longitudinal axis (A) of the cartridge ; a plunger carrier disposed for movement relative to the cartridge carrier , the plunger carrier being disposed to confront and impart movement to at least a portion of the plunger assembly ; at least one transfer instrument disposed to selectively couple the cartridge carrier to the plunger carrier for movement therewith, at least one of the plunger carrier and the cartridge carrier including an opening that selectively receives the at least one transfer instrument to removably couple the cartridge carrier to the plunger carrier ; an elongated drive device coupled to the plunger carrier , the elongated drive device being disposed to provide movement of the plunger carrier in a direction parallel to the longitudinal axis (A) of the cartridge ; a motor ; and a transmission assembly coupling the motor to the elongated drive device , characterised in that the cartridge carrier includes at least one opening , and the plunger carrier includes at least one opening , the cartridge carrier not be being coupled to the plunger carrier when the at least one transfer instrument is disposed in a first position within only one of the cartridge carrier opening and the plunger carrier opening , and the cartridge carrier being coupled to the plunger carrier for movement therewith when the at least one transfer instrument is disposed in a second position within both the cartridge carrier opening and the plunger carrier opening ; and further including a retainer disposed to selectively retain the at least one transfer instrument within at least one of the openings in either the cartridge carrier or the plunger carrier . a housing ; a cartridge carrier adapted to receive at least a portion of the cartridge , the cartridge carrier being disposed for movement relative to the housing in a direction parallel to the longitudinal axis (A) of the cartridge ; a plunger carrier disposed for movement relative to the cartridge carrier , the plunger carrier being disposed to confront and impart movement to at least a portion of the plunger assembly ; at least one transfer instrument disposed to selectively couple the cartridge carrier to the plunger carrier for movement therewith, at least one of the plunger carrier and the cartridge carrier including an opening that selectively receives the at least one transfer instrument to removably couple the cartridge carrier to the plunger carrier ; an elongated drive device coupled to the plunger carrier , the elongated drive device being disposed to provide movement of the plunger carrier in a direction parallel to the longitudinal axis (A) of the cartridge ; a motor ; and a transmission assembly coupling the motor to the elongated drive device , characterised in that the cartridge carrier includes at least one opening , and the plunger carrier includes at least one opening , the cartridge carrier not be being coupled to the plunger carrier when the at least one transfer instrument is disposed in a first position within only one of the cartridge carrier opening and the plunger carrier opening , and the cartridge carrier being coupled to the plunger carrier for movement therewith when the at least one transfer instrument is disposed in a second position within both the cartridge carrier opening and the plunger carrier opening ; and further including a retainer disposed to selectively retain the at least one transfer instrument within at least one of the openings in either the cartridge carrier or the plunger carrier . <EOS>