This brings me to my first point. The maximum amount of energy that can be transferred to a spud (in a theoretically perfect gun) is equal to the amount of energy stored in it's chamber.
A 1000cc chamber at 100psi has more energy than a 500cc chamber at 100psi.
A 1000cc chamber at 100psi has more energy than a 1000cc chamber at 50psi.
This is just common sense. More Chamber = More Energy
Now let's think about building a pneumatic spudgun. Most people start by choosing a certain size barrel and then ask "How big should my chamber be?" This is the same thing as asking "How much energy should I run through my barrel to get the best performance?" Common sense tells us that more energy will yeild more performance, and we already know that more chamber means more energy. We can logically assume then, that the gun will shoot the farthest with the biggest chamber we can find. This means that a gun with a 4:1 ratio will be more powerful than a 3:1, and a 6:1 more powerful than that, and a 10:1 more powerful than that, and so on as big as you want to go. Bigger is better.
FOR A PARTICULAR BARREL SIZE, THERE IS NO OPTIMUM CHAMBER SIZE
But hang on a minute! Let's say my buddy Bubba takes the chamber off his old gun and, after reading that last paragraph, attaches a 100 gallon propane tank in its place. His gun will be exceedingly powerful, but it will also be horribly inneficient. This is because Bubba's barrel isn't large enough to harness all of the stored energy, so most of it will be wasted as noise.
My second, more important point is that the barrel of a gun serves only one purpose. It converts energy stored in the chamber into linear kinetic energy of a projectile. A projectile accellerates down the barrel as long as there is energy available to push it. More Barrel = More Energy Conversion
If the projectile doesn't exit the muzzle before the available energy is expended, it will begin to decellerate. Thus, a long barrel will convert more energy than a short one until it converts all available energy.
If we rethink our spudgun with this in mind, things get more interesting. This time, let's start from the other end. If we choose a certain size CHAMBER to build around, we start with a certain amount of energy to drive our gun. Now we need to know how much barrel to use for maximum performance. The question we're asking this time is: "How much barrel do we need to transfer all the energy to a spud?" Here's a question we CAN find an actual answer to. If the barrel is too short, it won't harness all the energy, and if it's too long, it will start to slow the spud down again. Somewhere inbetween is where we'll find our maximum performance ratio.