internal barrel

[Rebelnerd]

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.

[spudshot]

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

[boilingleadbath]

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.

[Rebelnerd]

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

[taterjim]

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.