A sub-assembly of a medicament delivery device, the sub-assembly comprising a propellant source comprising a reservoir housing defining a reservoir and a stem having a bore therethrough, at least one inlet in fluid communication with the bore, and an open outlet end in fluid communication with the bore, the stem being moveable relative to the reservoir housing between a first position in which the at least one inlet is not in fluid communication with the reservoir and a second position in which the at least one inlet is in fluid communication with the reservoir, the propellant source further comprising a biasing member for biasing the stem towards the first position. The sub-assembly further comprising a collet member having a first plurality of radially flexible fingers and a second plurality of radially flexible fingers wherein the first plurality of radially flexible fingers extend radially inwards of the second plurality of radially flexible fingers when each of said first and second plurality of radially flexible fingers are unbiased. The first plurality of radially flexible fingers are configured to engage the stem so as to inhibit axial movement of the stem relative to the collet member when the stem is urged in an axial direction with a force less than a force threshold but permit axial movement of the stem relative to the collet member when the stem is urged in an axial direction with a force greater than or equal to the force threshold. The second plurality of radially flexible fingers do not prevent axial movement of the stem relative to the collet member when in a radially outward position but engage the stem when in a biased radially inward position so as to inhibit axial movement of the stem relative to the collet member including when the stem is urged in an axial direction with a force that is substantially equal to or greater than the force threshold and that is substantially equal to or greater than the force required to compress the biasing member
