In the 18th and 19th centuries, blackpowder cannons were elevated from crude, dangerous contraptions made in blacksmith shops, to precisely manufactured instruments of war. One problem, though, was that no matter how long the barrel of the cannon, designers couldn't raise muzzle velocities (and therefore, ranges) beyond a certain point. This was due to the limitations of their propellant. By comparison to modern smokeless powders, black powder detonates at a very slow rate. No matter how good the cannon, a projectile won't go faster than the explosion pushing it. Modern smokeless powders have much higher detonation velocities and therefore will push a projectile much faster.
The same effect is encountered with spudguns, which use relatively low-energy propellants like propane or compressed air. For instance, a mixture of Right Guard and air has a particular combustion velocity. A theoretically perfect gun with an optimized C:B ratio will get a spud moving as fast as that combustion velocity, but no faster. A perfect gun designed for propane will be capable of higher speeds, due to the higher combustion velocity of propane, but there will still be an upper limit to muzzle velocity, regardless of the size of the gun.
The same holds true for pneumatics. Air at 80psi can only travel so fast in an unrestricted pipe. Air at 100psi will only be able to go a bit faster. These velocities determine the peak performance possible for a gun.
This is all very logical and the conclusions would seem like no-brainers, but the point to remember is this: Once a gun's muzzle velocity equals the velocity of it's propellant, it can't get any more powerful, even if the chamber is 10 times larger, or the barrel 10 times longer.
This has relevance to a recent debate regarding the feasibility of a supersonic pneumatic gun. The question is whether or not room temparature air can be forced to flow at greater than Mach 1 by it's pressure alone. I honestly don't know but these are my thoughts until I verify them. Granted that with external forces you can easily accellerate an isolated column of air to supersonic speeds just like anything else, but a pressure wave inside that column of air will still travel at Mach 1 relative to the air. I recall that regardless of pressure, air molecules at the same temparature have the same kinetic energy and therefore the same individual velocity. Upping the pressure does nothing to the molecular kinetic energy, and so shouldn't raise the velocity of a wave through the air. So I suppose, unless you heated the air in the chamber, it would be impossible to go supersonic. But of course that's just me hypothesizing, I really don't know.