As Mark said, tight containment in metal is best. I use copper tubing and have had a number of successful flights. I would though recommend using Hodgdon Triple Seven FFFG instead of Pyrodex as it just burns better/faster.
i have tried both and did a little backyard testing.:y:
If I had a choice for ejection charge I would take the black powder. BP creates the desired pressure release for deployment. Pyrodex will work, I have used it a few times in deployment charges but it is not a recommended substitute for BP deployment.IMO