Can you use a floating head piston launcher (homemade) with a composite motor (AT F40W). We were all set to try making one when I found an article that mentioned in passing that you can't use them with composites. Is that true?
I would not recommend it. Composite motors, especially AT motors, have a tendency to chuff, which would extend the piston some but not launch the rocket at full thrust. The rocket would either fall off the rod right there, or else it would ignite fully when the piston is fully extended and be unstable.
You'd either need a piston that could vent a small amount of gas, or a motor that will always ignite instantly without chuffing. Cesaroni motors are close, but even so I'd say to have a piston with a long travel distance, some initial friction, and venting.