At Burnt Latke, you can find some great test data regarding C:B ratios. Latke used a small propane powered test gun and obtained an optimum ratio of about .8:1. While this did great things for spud theory, many people immediately latched on to .8:1 as the new magic number for combustion guns.
But why wouldn't it be valid for all guns? Well, there are many variables that determine performance. Projectile mass and barrel friction are two obvious ones. Chamber configuration and ignitor placement are others. One very important factor is energy density.
By energy density, I mean the amount of energy stored in every cubic inch (or cubic centimeter, or cubic whatever) of chamber. Different fuels drastically change the energy density of a gun. In his tests, latke ran his gun on propane. A chamber filled with propane contains much more chemical energy than an identical chamber filled with Aquanet. As we saw before, this means the volume ratios will be different for peak performance. A .8:1 ratio would be great for a gun similar to latke's with similar ammo and similar fuel, but for other guns, ideal performance ratios can be much different. For instance, say you design a gun to be optimized for Right Guard. Then say you convert it to propane. There is now more energy in the chamber, which will require a longer barrel to harness
This is also true for pneumatic guns. For example, imagine two identical pneumatic spudguns. If Gun A is filled to 70psi, and Gun B to 110psi, the guns do not contain the same amount of energy, even though they both have the same size air chamber. It will take more barrel volume to optimize Gun B because it contains more energy. As this illustrates, even guns with identical chamber volumes and valve configurations can have different optimum ratios, depending on usage.
To sum all this up: Guns With Higher Energy Densities Need Longer Barrels To Be Optimized.
One thing this means is that, for pneumatic shooters, changing chamber pressure from shot to shot will change the barrel size needed for an ideal ratio. This is one reason why I began by saying that it is rarely practical to optimize a gun. If you design a gun specifically for 100psi, but sometimes shoot it at 60, the barrel will be too long for the lower pressure. This is not a terrible thing, but it would probably be better for less specific applications to find a happy medium.