bravootome wrote:actualy is not so hard to get supersonic, it's all about pressure.
Think about what the definition of the speed of sound is. To sound like Captain Obvious, it's the velocity at which sound travels in a material.
What is sound? The propagation of pressure through a material.
Hence, rephrased, the maximum speed at which pressure can propagate through a material is its speed of sound.
No pressure, no force, no acceleration. Hence, a gas's expansion is limited to its speed of sound. The only way to get supersonic with a pneumatic is for the launcher's internal speed of sound to somehow be higher than the external speed of sound.
Now, the speed of sound is proportional to the square root of gas pressure, but it's also inversely proportional to the square root of gas density - and the ratio of pressure to density for a given gas is defined solely by temperature. Hence, the starting pressure is of no relevance to the speed of sound.
So, with air, the internal speed of sound will only exceed the external one if the temperature of the gases is higher than ambient temperature. This is not generally the case, and the adiabatic expansion of gas actually results in a fall in temperature.
Hence, it's genuinely difficult to go supersonic with a pneumatic. There are ways it can happen, of which the most plausible is shock heating of the "dead air" between the valve and projectile allowing the temperature in the barrel (and thus speed of sound) to rise above that of the external environment.
But that's hard to get right. So, as a general rule, if you think you've gone supersonic with a pneumatic, you're probably mistaken.