Side curtain air bag assembly

An inflator (22) for providing inflation fluid () to inflate an inflatable vehicle occupant protection device (14) includes a container (30) storing the inflation fluid under pressure. The container (30) has an outlet passage (80) through which the inflation fluid flows from the container. A rupturable closure member (92) fixed to the container (30) blocks flow of inflation fluid through the passage (80). A support (100) for the rupturable closure member (92) defines a chamber (110) adjacent the rupturable closure member (92). The rupturable closure member (92) has a first portion (122) deformed into the chamber (110) by the pressure of the inflation fluid and a second ring-shaped portion (124) encircling the first portion (122). An initiator (98) ruptures the closure member (92) when actuated by shearing the first portion (122) from the second portion (124).