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these are some of the photos i screenshot from my CAD program.
heres my idea, the barrel is pretty much the length of the cannon, the chamber is surrounding it with an endcap on the end the barrel extends out(as seen on bottom right pic of viewport). these gasses will have no other choice but to expand in the opposite direction of the potato's trajectory and then expand into the barrel direction(as seen on top right pic of view port).
this is probably very confusing, but i hope the pic helps.
i got this idea from how a space shuttle boosters work.
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thats how i got the idea.