You're both right, at the boundary layer between the volume of air you're pushing and the tunnel there is going to be a bit of turbulence given the viscosity of the air, within the bubble you may never exceed the mach number for the air around you. Excellent paper put out by Barrett on designing long range sniper rifles [1] talking about the issues have having the bullet not be deflected by leaving the barrel (where even though its travelling quite fast it has yet to exceed the mach number of the expanding gas around it) and the ambient atmosphere (where the air is 'still' to the now speeding bullet). Needless to say the paper lost me in the fluid dynamics of the barrel air mixing with the outer air but they seemed pretty clear that inside the barrel things were pretty linear up to that point.

Anyway, what ever it turns out to be I'm sure it will be cool.

[1] My Dad is an engraver with an FFL and gets all sorts of interesting things in the mail. Usually to engrave, but the promotional literature is cool too.