The difference between a flush bolt and a shootbolt
You will usually find a shoot bolt fitted to the edge of a slave door on a set of double doors. To operate a shoot bolt, it first needs to be connected to a second device, such as a gearbox or lock. When a shootbolt is operated, a small pin extends into a recess in the floor. Not only does this keep the door securely in place, but also prevents the master door lock from bending and damaging both doors over time.
A flush bolt is a similar device to a shoot bolt, except that the fixing bolt has a long rod instead of a pin, that extends into the recess. This is an entirely independent device and it does not need to be connected to any other mechanism to function. A flush bolt can be found on aluminium doors, such as automatic doors.