Disclosed is an injection device for delivering a medicament from a syringe contained within a housing, the device being configured to move the syringe through the housing to cause insertion of a needle of the syringe into a user's skin and move a syringe bung through a syringe body, delivering medicament through the needle. The device includes: a plunger engaging the bung； a sleeve around the plunger； an insertion spring biasing the sleeve to the housing； and a delivery spring biasing the plunger to the sleeve. A release mechanism releases the sleeve from the housing to commence insertion and release the plunger from the sleeve, following insertion, delivering medicament. A velocity regulator regulates the plunger upon release and disengages during delivery, and includes a screw thread associated with the sleeve or plunger and one or more thread engaging members formed on the other of the sleeve and the plunger.
