SpudFiles

What you would need:
-coupler (matched to fit barrel)
-spring
-Plastic/Acrylic/Wood/Steel (doesn't matter, for the folding arm and as well as a spring mount)
-Glue or Epoxy
-3x Bolts, washers and nuts

Process of how it works:
-Push coupler back and tilt the barrel to gain access to the breech, hence being able to load easily and fast.
-Tilt barrel back down and release coupler so it locks the barrel and opposite breech end (from the tee/elbow, or whatever you may be using).

Other things to mention:
-The folding arm I think is neccessary as both a barrel support and way to guide the barrel. It simply attatches from the barrel to chamber, using some easy homemade brackets with some glue.

**more to come**
Hopefully on my next design, and feel free to be a critic on this.

looks like it would work well, but I'm new to this stuff so I don't know

Is this a break-barrel design, such as in Goat's Gun? If so, why not use a hinge instead of the plastic/acrylic/wood/steel?

I think it will have a good seal and should be easy to use. Now, to make it take shells...

Yes like that cannon-
Funny thing is after I posted this I seen that one by accident. I even looked before hand to see if anyone had done it before.