The chamber isn't pressurized unless you're building a pneumatic or a hybrid (not reccomended for a newb). The fact that the chamber is at atmospheric pressure means that there is very little force wanting to make the gas leak out, so the projectile is more than enough to seal it up.

I reccommend ...

Ok, i am trying to figure out the dynamics of one here.

what keeps the propellant from exiting the chamber prior to being ignited?

the potato? if i used propane, would i need a stop valve behind the potato or is the potato enough to keep pressure in the chamber?

well, this will be my first one, but i dont plan on building a little one, i like to go all out. If i went with a combustion using one of those coleman fuel tanks, what sort of setup would i need?

I have surfed a lot here and much of the combustion section is a bit advanced for me as of now ...

I've been looking for a release........ and i think i've found it.

I am a gun nut, but there is something about hurling large projectiles long distances with stuff I can buy in Home Depot that puts a smile on my face.

I want to construct my first launcher. I have a general idea, but plan to use ...