SpudFiles

would it be ok to make a cannon with a long combustion chamber, and the barrel inside it, but not quite touching the back? it would save space, but i'm worried it may damage the barrel.

that is a coaxial type cannon, yeah its fine, just give some room in the back.

And some sort of holder in the back (and middle) to keep it centered.
At the very least, going adding support fins is an accuracy improver - less barrel flex because it's stabilized by the stiffer large pipe (should improve it, anyway - to my knolage, no tests have been done on it). To maximize this effect, I reccomend epoxy-ing the rear fins in place. (makes it quieter, too - less rattling)
At most, the fins keep you safe by preventing said flexing barrel from hitting the side of your chamber, and causing failure.

ok. and would PVC be able to withstand the conditions of being a barrel in this type of gun?

I asked this question under a different topic a few weeks ago.and aslo if anyone had ever built a combustion out of anything like a poly spray bottle or maybe a thick jug or something.I may try the barrel inside thing with one of my next cannons as I plan to build a few just to try out such Ideas. At this time I have put a tee in one of my combustions to see if rear of center fire works the best. I will post that info as well as a picture of the remodeled cannon in a few days. One thing I did learn that I can pass on is that a 2" pvc male fitting can be pressed on a 1 1/2" pipe as I have done that. It is so tight it take a hammer and block to move it. I did this so I could thread on a two inch pipe over a one an a half and it worked well.