An assembly for a firing mechanism for use in an injection device and comprising: a rear cap (104, 602, 802) having an elongate member (110, 804, 904, 1004) extending axially forwards when installed within the injection device; a plunger carrier (810, 910, 1010) releasably connected to the elongate member (110, 804, 904, 1004) and fixed in position relative to the elongate member (110, 804, 904, 1004) before release from the elongate member (110, 804, 904, 1004); a plunger (106, 604, 806, 906, 1006) releasably connected to the elongate member (110, 804, 904, 1004) via the plunger carrier (810, 910, 1010) and, upon release of the connection between the plunger carrier (810, 910, 1010) and the elongate member (110, 804, 904, 1004), axially displaceable in a forward direction; and a plunger driver (112, 1008) to drive the plunger (106, 604, 806, 906, 1006) axially forwards upon release of the connection between the plunger carrier (810, 910, 1010) and the elongate member (110, 804, 904, 1004), the plunger driver (112, 1008) positioned between the rear cap (104, 602, 802) and the plunger (106, 604, 806, 906, 1006) and received within the plunger (106, 604, 806, 906, 1006) such that a first end of the plunger driver (112, 1008) abuts a forward end of the plunger (106, 604, 806, 906, 1006), wherein the plunger (106, 604, 806, 906, 1006) and the rear cap (104, 602, 802) define an axial length of the assembly, which in turn determines a start position of a forward end of the plunger (106, 604, 806, 906, 1006) before release of the connection to the elongate member (110, 804, 904, 1004), wherein one or both of the plunger (106, 604, 806, 906, 1006) and the plunger carrier (810, 910, 1010) comprises a plurality of connection features for connection between the plunger (106, 604, 806, 906, 1006) and the plunger carrier (810, 910, 1010) during construction of the assembly, and wherein, during construction of the assembly, the plunger (106, 604, 806, 906, 1006) is releasably connectable to the plunger carrier (810, 910, 1010) at one of a plurality of positions defined by the plurality of connection features for controlling the axial length of the assembly. a rear cap (104, 602, 802) having an elongate member (110, 804, 904, 1004) extending axially forwards when installed within the injection device; a plunger carrier (810, 910, 1010) releasably connected to the elongate member (110, 804, 904, 1004) and fixed in position relative to the elongate member (110, 804, 904, 1004) before release from the elongate member (110, 804, 904, 1004); a plunger (106, 604, 806, 906, 1006) releasably connected to the elongate member (110, 804, 904, 1004) via the plunger carrier (810, 910, 1010) and, upon release of the connection between the plunger carrier (810, 910, 1010) and the elongate member (110, 804, 904, 1004), axially displaceable in a forward direction; and a plunger driver (112, 1008) to drive the plunger (106, 604, 806, 906, 1006) axially forwards upon release of the connection between the plunger carrier (810, 910, 1010) and the elongate member (110, 804, 904, 1004), the plunger driver (112, 1008) positioned between the rear cap (104, 602, 802) and the plunger (106, 604, 806, 906, 1006) and received within the plunger (106, 604, 806, 906, 1006) such that a first end of the plunger driver (112, 1008) abuts a forward end of the plunger (106, 604, 806, 906, 1006), wherein the plunger (106, 604, 806, 906, 1006) and the rear cap (104, 602, 802) define an axial length of the assembly, which in turn determines a start position of a forward end of the plunger (106, 604, 806, 906, 1006) before release of the connection to the elongate member (110, 804, 904, 1004), wherein one or both of the plunger (106, 604, 806, 906, 1006) and the plunger carrier (810, 910, 1010) comprises a plurality of connection features for connection between the plunger (106, 604, 806, 906, 1006) and the plunger carrier (810, 910, 1010) during construction of the assembly, and wherein, during construction of the assembly, the plunger (106, 604, 806, 906, 1006) is releasably connectable to the plunger carrier (810, 910, 1010) at one of a plurality of positions defined by the plurality of connection features for controlling the axial length of the assembly.