Mods, if this isn't the right place go ahead and move it and it's cool if you decide to close the non-project mortar thread I made.
So I decided to try and make a dual-purpose Mortar/Handheld Bazooka (thanks for the idea Shady

).
All I really need to know is how to make it so it holds pressure. I searched around and couldn't find anything that really helped. I know that it needs a pressure chamber and a valve system, but what materials should I use? How should I use them?
Thanks in advance

!!