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You really didn't describe the inner workings of the cannon much, like what are you using for a chamber, why all the extra length of rubber hoses and what is your rationale for using propane as your propellant as opposed to CO2 or HPA.
I assume the purpose of the fake backpack is to balance and have something to bear the weight of the cannon itself. Also, the arm gauntlet seems overbearing and overly complicated for the few operations it performs...it does look cool though...
Sorry, I don't mean to seem totally negative on the cannon. You obviously have some skills in fabrication and a reasonable knowledge of pneumatic cannonry. It's just that for all its complexity and bling factor, there is really little to recommend it from a practical or performance perspective.
One practical issue is loading it. Since you can't reach that far, you'll have to take the barrel off to muzzle load each shot....or...have a buddy do it for you.
Edit: Well in looking again,
maybe you can just barely reach that far....
