A syringe plunger/jacket assembly may comprise a flexible jacket attached to a rigid plunger. The jacket may comprise at least two sealing rings (front and rear) that prevent fluid from leaking out of the rear of the barrel during injection of fluid from the syringe. The rear sealing ring may not contact or seal the interior surface of the syringe barrel in an unloaded state. In the loaded state, when force is applied in the direction of the forward discharge end of the syringe, pressure is created on the jacket. This condition causes the rear seal to expand and contact and/or seal the interior surface of the syringe barrel.