An automatic injector for delivering a pharmaceutical substance from a reservoir of a cartridge, the cartridge interconnected to a needle at its distal end and mechanically coupled to a plunger driver at its proximal end, the injector comprising a connecting feature on the cartridge a contact surface for contacting an injection site a frame connected to the contact surface, the frame having an interference element with a distal stop, the interference element sized to slidably link to the connecting feature and allow the cartridge to slide up to the distal stop, a snap element allowing the interference element to receive the connection feature, but inhibiting disconnection of the cartridge; the frame further comprising a driver mount, for fixedly containing a plunger driver, the driver for driving a plunger to empty the reservoir of the cartridge through the needle to inject fluid from the reservoir into the injection subject.