I think that maybe you should first learn how to read the numerous posts informing you of places you can research and answer these questions yourself. You are essentially asking the same questions that you have asked from the beginning, and the answer will remain the same: Figure it out yourself!
From my knowledge with the launcher i have built, powered by compressed air.
Is there something blocking the chamber from the barrel?
Knowledge? How the hell do you think it would work otherwise? You will get much more respect here if you try to figure some things out for yourself, this is just showing that you can't even use your intellect.
The proper ratio of Chamber to Barrel in a combustion is about .8:1, which, if you can be bothered to comprehend it before making another mindless post, should answer your questions of chamber and barrel length.
A stoichiometric mixture of C3H8 and air contains about 4.2% propane and 95.8% air. In an airtight chamber, adding the right amount of fuel would add around 0.6psi, which is why fuel meters are used.
This amount of propane should produce the highest pressure possible (which will be around 85psi max if you have an accurate fuel meter. In short, as long as you use pressure rated PVC, you don't have anything to worry about with combustion launchers.
Please don't ask reiterations of the previous questions again, just do some research.