Caution! Now this is BEING Built. It has not been FIRED yet. So proceed at your own RISK.
Update: It has been built. Link at http://www.pbnation.com/showthread.p...0#post69862960
I will add a test report when completed but I am confident it will get the job done.
All of us Cannoneers tend to overbuild our breechloaders. My self included. Must be a Guy Thing or fear of needing plastic surgery.
Actually, there is suprisingly little pressure build up in the barrel during firing. The Nerf is So Light, pressure doesn't build. My Shorty which operates at the HIGH END of most cannon operating pressure at 145 psi generates a max pressure of 40 psi for the first milisecond and the graph drops exponentially to ZERO over the next four miliseconds when the Nerf exits the barrel. That is from Dave Halls Gas Gun Design Tool. In the field, I discovered you can hold your bare palm over the breech of a cannon and FIRE a Nerf that goes screaming out the barrel. Just a Big Noisy Puff, not a BLAST of air.
Here is a couple of pictures of my first overbuilt Breechloader.
Clamps on top and bottom and wedges tight on those tapered angled aluminum.
I am building a Hand Held Breech Loader using this PVC DRAIN from LOWES.
Bought the one that had the beefest screw holes to secure the breech plate and would accept 2 or 3 inch PVC pipe.
First wrinkle is the Nerf would be a bit too far back relative to the air Inlet hole.
So you could cut off some of the T fitting and/or the Drain. I opted for the drain because it was rather thick walled. Remember you can fudge like this because there is NOT a lot of pressure build up during firing. Unless you forget to take out your barrel plug and didn't vent it just in case you DID forget.
This solved the Nerf loading problem.
For the breech plate, I'm going with 1/4" plywood.
Here is a pic of a mock up.
You will note the drain had a ridge that has to be reamed out to accept a nerf easily. You can see the slot offers a source for air to leak out. Again, as long as it leaks the same amount, it doesn't matter. However, Since that ridge is going to be removed anyway, I plan to extend the 2" pvc pipe rearwards about 1/8"so that the end of that 2" pipe is FLUSH with the breech plate to minimize the leak potential. ANOTHER really Good reason to extend that 2 inch pipe is the FORCE exerted on the breech plate. 40 psi x the area of that 4" drain is 502 pounds of force. That got MY attention even IF it is for only 1 milisecond. However that same 40 psi on a 2" pipe flush agaisnt that plate is 125 pounds of force. 75% less force on that breech plate and I wouldn't MIND if any leakage out of the 2 inch pipe is vented out that slot instead of building up. It is a good idea to pay attention in physics class...
Builders also worry about sealing the breech. Making it air tight. It is NOT critical and can leak a bit. What is critical is that it leaks the SAME amount of air on each shot so accuracy/range is not affected.
In the FIRST breechloader I used a coarse thread Machine Screw for the pivot. (4-20) and there was NO pivot. The bolt was fixed and attached to the breech plate that moved and was threaded into a nut fixed to the cannon plate. So when I opened the breech a quarter turn, the breech plate literally unscrewed a quarter turn and OPENED a small gap between the cannon and breech plate. Enough to allow it to open easily and tighten up when closed.
Now this is BEING Built. It has not been FIRED yet. I will add a test report when completed but I am confident it will get the job done. Something to ponder for now...