Abstract: An apparatus comprises vehicle seat belt webbing (18), a cylinder (50), and a piston (60) movable in the cylinder to effect pretensioning of the seat belt webbing. The apparatus also comprises a fluid source (10) for providing fluid under pressure to move the piston (60) in the cylinder (50). The fluid source (10) comprises a propellant container (122) defining a propellant chamber (123) and a propellant (82) in the propellant chamber. The propellant (82) is ignitable to produce fluid under pressure. The propellant container (122) comprises a first portion (124) made from plastic and a second portion (126) made from metal. The metal portion (126) is secured to the plastic portion (124) to effect a hermetic seal of the propellant container (122).