Here's my latest mini spudgun built as a miniature replica of the cobra venom by Moonbogg.
Barrel: 12.5 cm long, 3.5mm bore
Chamber: 2.5 cm long, 9mm i.d (it is longer, chamber plugs occupy a large amount of space)
Over/under construction with chamber brace and fore barrel support.
Chamber plug is threaded and has a built in check valve
2 spark gaps
Propane, MAPP or oxygen-hydrogen fuel (metered)
Me making miniature replicas on request:http://www.spudfiles.com/forums/viewtopic,p,250552.html#250552
Inspiration, the Cobra Venom by Moonbogg:http://www.spudfiles.com/forums/the-cobra-venom-t17948.html
Will I do an instructable? Depends. If someone suggests a good cannon for me to replicate (on SF) I might.. make one. Though these are very intricate, requiring a lot of understanding, planning and determination..
Also, my camera crapped itself so photos are taken with phone.