Many of us now use a short tube (4"-6") on top of our piston tubes creating an air gap between the rocket motor and the piston head. With black powder motors this is less for performance than it is for ease of use with fixed head pistons. Added benefit is that most of the soot from the motor ends up in the buffer tubes making repeat use of the longer piston tubes more possible.

You can piston launch composite motors, like the Quest Q-Jets , or other Aerotech single use motors if you use a buffer tube to prevent chuffing. Without such a buffer tube composites have difficulty igniting in a zero volume piston.