A plunger-barrel apparatus is adapted for use within a syringe-in-syringe mixing system for mixing a two-part composition (e.g., a dental composition). A hollow plunger-barrel body has a continuous cylindrical wall defining an internal chamber for containing a first component. The body includes a plunger receiving end and a dispensing end. An integral sealing plug and rupturable membrane is disposed at the dispensing end of the body and include a sealing plug portion and a rupturable membrane portion that are integrally formed together as a single piece (e.g., formed of a single piece of elastomeric material). An associated syringe-in-syringe mixing system includes a first plunger, the hollow plunger-barrel body as described above adapted to contain a first component, and a syringe barrel configured to contain a second component. When assembled, the first plunger is slidably disposed within the hollow plunger-barrel body, and the plunger-barrel body is slidably disposed within the syringe barrel. The two components are initially separated by the rupturable membrane.