An injector includes a trigger mechanism including: a trigger member disposed about an axis having an aperture and a protrusion, and a ram assembly having a ram configured to pressurize a medicament container for expelling a medicament therefrom, the ram assembly further having a trigger engagement member configured to engage the aperture of the trigger member when the trigger member is in a pre-firing condition； an energy source associated with the ram for powering the ram to expel the medicament； and a user-operable firing-initiation member having an aperture engaged with the protrusion of the trigger member and operable for causing an axial translation of the trigger member in a proximal direction from the pre-firing condition to a firing condition in which the trigger engagement member is released from the retaining portion to allow the energy source to fire the ram.
