But you're limited by the mach speed of the fluid pushing the object, not the mach speed of the fluid you're traveling through. A bullet out of a rifle is limit

But you're limited by the mach speed of the fluid pushing the object, not the mach speed of the fluid you're traveling through. A bullet out of a rifle is limited by the mach speed of the hot gases created during combustion. A compressed-air gun, however, is limited to the speed of sound in compressed air.