Abstract: An automatic injector comprises a housing, a tubular cartridge disposed in the housing, the cartridge having a rearward portion thereof with a predetermined inner diameter and an inwardly extending shoulder connecting a forward portion of the cartridge with a smaller inner diameter than the predetermined diameter. A charge of liquid medicament is normally disposed in the cartridge, and a needle is disposed within the housing and adapted to communicate with the medicament through a forward end of the cartridge to dispense the medicament therethrough. A plunger disposed within the cartridge and rearwardly confines the medicament and has a rearward portion with a diameter sized to form a seal with the predetermined inner diameter of the rearward portion of the cartridge. The plunger is movable forwardly through the cartridge in slidingly sealed relation with respect thereto to displace the medicament through the needle.