Auto-injector with uniform pressure exertion of a primary container

Parenteral delivery of a beneficial agent using an auto-injector configured for reduction of forces acting on a primary container for the beneficial agent during activation of the auto-injector. The primary container may be disposed within a pressure chamber of the auto-injector such that an increased pressure within the pressure chamber during activation is applied to a stopper of the primary container and an external surface of a sidewall of the primary container. In turn, a pressure differential between a containment volume of the primary container and the pressure chamber may be reduced or eliminated. In turn, forces acing on the primary container in response to the pressure increase in the pressure container may be reduced or eliminated. Configurations are also provided for improved relative movement between a stopper and a sidewall of the primary container for more reliable injection.