A conventional shock tube was used to accelerate a preignited flame bubble from a circular pipe into a larger vessel. The diffraction of the ensuing shock-jet flame at the abrupt area change was observed by spark and streak schlieren photography. The results confirm previous findings that jet initiation of detonation can occur at initial conditions where an established detonation would fail to transit into the unconfined volume. The schlieren observations also show that significant unconfined flame acceleration can occur in the larger volume. Transition to detonation was always observed to occur in close proximity to the surface of the diffracting turbulent flame front and the results do not favor a transition mechanism involving extended vortex roll-up and larger scale mixing of burned and unburned gas.