DYI wrote:the maximum attainable velocity is not c_0 (the initial sound speed), but rather 2c_0/(γ-1)
I know (but the equation's actually in Section 11), but there's two problems with it in this context.
Firstly, it makes a fairly large number of assumptions that are inaccurate when applied to homemade pneumatics (of a relatively low pressure). For our purposes, Mach 1 is pretty functional rule of thumb.
Secondly, delving into all the depths of fluid mechanics is a horrendously over-complex way to try and explain that breaking the sound barrier is not quite so trivial that everything you think might be a sonic crack is.
Sometimes, the dumbed-down explanation is the stronger choice.