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12-26-98... I have decided that I will probably never build this gun. But I have decided to leave it here to give ideas and inspiration to others that may wish to design and build a large co-axil type air cannon.
Also note that the piston setup in this cannon will never let the piston unseal the barrel to fire. This is be cause the surface area of the piston seal is larger than the surface area of the piston end seal that is exposed to the pressure in the main air chamber
Enjoy,
Patrick.

Hi Guys,

As some of you know ,some of us have been working on building really BIG air cannons. I have decided to stop working on my first design.... the one with 8 air chambers feeding into the barrel. I don't think there was a problem with the design just a problem with assembling it properly so as to not have an air leaks. By the very nature of the design I could not rotate many of the parts during final assembly< which is necessary for a good air tight connection, especially with large parts i.e.. 4" pipe.> and sense there would be over 200 connections , I just could not see this thing not have some air leaks.

So the above picture is the basics of my new design< far fewer parts>.

Note: New changes not shown in drawing.... the 4"x8" bell adapters will be replaced by 4"x8" bushings attached with 8" pipe couplers. Also with the piston of this design , the gun will NEVER fire. A new piston design is needed.

It is pretty much to scale with the exception of the 4" barrel length and the 8" air chamber length
Note that not shown are:
The wiring and batteries.
Mounting supports, to attach to a stand.
Steel flashing, that will be wrapped around the outside of the 8" air chamber and 8"x4" adapters. This will be held in place by large hose clamps.< I have discovered that these are deceptively strong for there size when tightly clamped down.>
When putting on the clamps I will put 4 of them at each joint on the adapters. this is to ensure no leaks< I hope> as I think there will be some swelling on the gun parts when pressurized, not much but maybe enough to cause a leak at the high pressure.
I may change the design of the piston slightly... the long length is required to keep it aligned correctly as it sticks out of the short piece of pipe on the rear of the gun.
I will put a threaded adapter on the end of the barrel to add length to it and to adapt it down to smaller barrels.
I know the 3"x4" adapter will cause a small restriction in air flow , but with the weight of a 4" projectile, I don't see this as much of a problem.
I will use 60" air chamber and 60" barrel, I would love to use a full 10ft. for both but that would be too large to move around, we'll just have to see how that goes when <if>I start building it.
A 4" barrel with 8" air chamber will give a barrel to air chamber ratio of 2.73 to 1 approx..
A 3" will give 4.86 to 1
A 2" will give10.93. to 1
Note that this is with the 4" barrel adapted down to the smaller barrels inside the 4" barrel.

I am open to suggestions

Patrick Parsons.
ps. the drawing would have been clearer , but I had to clean it up with an eraser after my 2yr. daughter helped me with it when my back was turned.