Thank you to "The Powers That Be" for setting this up. As suggested in the meeting, I'll start with my new love of piston ejection.
I've always been a Baffle Guy. If the tube was BT-60 or above, it got a baffle. Baffles do have their issues. They don't work in small tubes. Even after shaking the tube out, it is surprising how much build-up can occur. And after a couple dozen flights, the tube can degrade where the baffle is, from the hot debris constantly stopping in the same area (I've yet to launch the same rocket a couple dozen times).
Then I built my first high power kit, an AIM-120 from PML that came with 4 inch plastic body tubes and piston ejection. In building the piston system, I saw how easy it would be to apply to even the smallest of body tubes. I had four rockets set up with pistons at the club launch on the 7th, from BT-50 up to the 4 inch AIM-120, and all four worked great.
The basics of a piston is a kevlar cord with one end at the motor mount and the other at the piston. The Cord comes up through the middle of the piston bulkhead. Form a loop and a knot in the cord so that it can't get back through the bulkhead and glue or epoxy to help. The cord should be as long as the shock cord as it needs time to slow down in open air from the ejection thrust. The shock cord is attached to the loop that was made.
For BT-50, BT-20 and BT-5, use a spent Estes engine of the right size for the piston. Other than that, any coupler will work. The bulkhead should be wood. Glue/epoxy the crap out of the bulkhead to piston connection, but be careful to keep the piston clean. There are tricks for sealing and sanding paper, if you really want to get deep.
Things that would stop you from using a piston are obstructions inside the tube, such as the Estes paper shock cord mount or beads of glue running down the inside of the tube from over-gluing the motor mount into place.
With piston ejection, you want to keep things clean. After launches, I run a paint-gun cleaner down through the rocket. I examine and clean the piston, if necessary. I wont hesitate to use a little light sandpaper on the piston. It should be as smooth to a lot smoother then the nosecone.
Attached is a pic of my latest build, a BT-50 sized WAC Corporal with a "D" Size Piston.