Injection device fill volume management

An apparatus for a firing mechanism for use in an injection device and including: a rear cap having an elongate member extending axially forwards when installed within the injection device; a plunger releasably connected, directly or indirectly, to the elongate member and, upon release of the connection to the elongate member, axially displaceable in a forward direction; and a plunger driver to drive the plunger axially forwards upon release of the connection to the elongate member, wherein the plunger and the rear cap define an axial length of the assembly, which in turn determines a start position of a forward end of the plunger before release of the connection to the elongate member, and wherein, during construction of the assembly, the plunger is releasably connectable, directly or indirectly, to the elongate member at one of a plurality of positions for controlling the axial length of the assembly.