Piston Valve or Quick Dump Valve ?

[Ranger]

For my next project I want to build a big Gun, 2" bore and 3" chamber. All this time I had been planning on making a Coaxial Piston Valve cannon, but then I found this
http://www.spudfiles.com/forums/quick-d ... 17858.html
Technician1002's marshmallow cannon and his T-Shirt cannon. I like both designs but I'm trying to figure out which one would be better, I'm going for distance and power so I want the better of the 2 designs.

I cant figure out which one would be better, it would make sense that the coax would have the highest flow rate (since its opening the entire back of the pipe not just little ports on the side) but the QDV could be better becuase of a quicker opening time (or someting that I havent thought of)

Any thoughts on the subject?

[Technician1002]

Those little ports have more cross section area than the pipe it feeds into. Check the details.

3 ports 1 inch by 2 inch is 2 square inches per port. That is 6 square inches total. The cross section area of the 2 inch pipe is about 1/2 that area at 3.14 square inches.

The t shirt launcher was launching into the 300 section (nosebleed seats) on only 30 PSI.

The Coaxial piston is easier to build. The QDV takes more work getting a good sealing surface and it seals in 2 places so piston construction is much more complicated This is not a project for a first time builder.

The ABS version instead of the steel version is much less difficult to construct. Use an adequate bumper if making it from PVC. The piston slams open hard.

My most recent damage photo is from today. An empty pop can was launched into a steel plate.

May I recommend a 4 inch chamber instead of 3 inch? My ABS cannon was constructed this way.

limited power is not an issue.

[Gun Freak]

I recomend a co-ax because of the ease of construction and how little parts you need. I have never made a QDV myself, but it really depends on the tools you have to work with and the amount of work you plan to dedicate to it.