A cassette unit (1; 10; 201; 301; 501; 601) for use with an auto-injector having an electrically powered drive unit, said cassette unit comprising: a cassette unit housing (20; 120; 220; 320; 520; 620) defining a cassette unit housing cavity and a needle projection aperture; said cassette unit housing cavity in receipt of a syringe (10; 110; 210; 310; 510) comprising: a barrel (12; 112; 212; 312; 512) for containing a volume of a liquid drug formulation, said barrel defining a flange at the rear end thereof and a forward shoulder at the forward end thereof; a hollow needle (14; 114; 214; 314; 514) at a front end of said barrel, said hollow needle defining a needle tip for dispensing of said liquid drug formulation; and a plunger (18; 118; 218; 318) that is axially movable within the barrel; and in contact with said plunger and axially movable within the barrel, a plunger slaving part (84; 184) arranged such that when a drive load is applied to a rear drive-receiving face thereof the drive load is evenly transmitted to the plunger, characterised in that a drive end of a drive transfer element (83c; 187), for transferring axial drive to the plunger slaving part and subsequently to the plunger, is arranged for receipt in a central recess (93; 193) of the plunger slaving part and configured to apply a point load to the rear-drive receiving face of the plunger slaving part and wherein said drive end defines a conical tip and said central recess is of conical form to guide and centre said conical tip therein. a cassette unit housing (20; 120; 220; 320; 520; 620) defining a cassette unit housing cavity and a needle projection aperture; said cassette unit housing cavity in receipt of a syringe (10; 110; 210; 310; 510) comprising: a barrel (12; 112; 212; 312; 512) for containing a volume of a liquid drug formulation, said barrel defining a flange at the rear end thereof and a forward shoulder at the forward end thereof; a hollow needle (14; 114; 214; 314; 514) at a front end of said barrel, said hollow needle defining a needle tip for dispensing of said liquid drug formulation; and a plunger (18; 118; 218; 318) that is axially movable within the barrel; and a barrel (12; 112; 212; 312; 512) for containing a volume of a liquid drug formulation, said barrel defining a flange at the rear end thereof and a forward shoulder at the forward end thereof; a hollow needle (14; 114; 214; 314; 514) at a front end of said barrel, said hollow needle defining a needle tip for dispensing of said liquid drug formulation; and a plunger (18; 118; 218; 318) that is axially movable within the barrel; and in contact with said plunger and axially movable within the barrel, a plunger slaving part (84; 184) arranged such that when a drive load is applied to a rear drive-receiving face thereof the drive load is evenly transmitted to the plunger, characterised in that a drive end of a drive transfer element (83c; 187), for transferring axial drive to the plunger slaving part and subsequently to the plunger, is arranged for receipt in a central recess (93; 193) of the plunger slaving part and configured to apply a point load to the rear-drive receiving face of the plunger slaving part and wherein said drive end defines a conical tip and said central recess is of conical form to guide and centre said conical tip therein.