An apparatus includes a movable member and a valve coupled to the movable member. The movable member is configured to be disposed within a housing of a medical device and has a first end portion and second end portion. A portion of the first end portion is configured to define a portion of a boundary of a gas chamber. The first end portion defines an opening configured to be in fluid communication between the gas chamber and an area outside the gas chamber. The second end portion is configured to be coupled to a needle configured to deliver a medicament into a body. The valve is configured to selectively allow fluid communication between the gas chamber and the area outside the gas chamber through the opening defined by the first end portion of the movable member.