Update:
I bought some pipe and made barrel extension; I sanded down the division in the coupling so there would be no space between the two pipes. It has now a 3ft barrel and a 0.75 C:B ratio.
Here is a damage picture using a PVC dart against a 1/4" pine board:
It is made from sch 40 PVC. The chamber is 6.357" of 2" pipe and the barrel 13" of 1" pipe. The C:B ratio is 2.76:1; it is not very good since I made the barrel from a small scrap piece of pipe I had. The cannon has a 1.5" computer fan that I power with a 9v battery. It has a BBQ piezoelectric lighter ignition.
Those are the fuels I use. Either metered butane from lighters with a syringe (I use 11mL of butane) or one pump of 99.8% isopropyl alcohol from a nasal spray bottle works very well. After combustion using the alcohol there is not residue or liquid on the chamber since it all evaporates. With the butane I get very consistent shots; it rarely misfires. I can't say about the alcohol since I mostly use the metered butane.
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