Shotgun microphones are just ordinary directional microphones at low and middle frequencies. The perforated tube in front of the capsule has little or no effect when the wavelengths are long. It's only at upper-mid and high frequencies (the frequencies that determine the intelligibility of speech, by no coincidence whatsoever) that shotguns have narrower pickup patterns than conventional microphones.
The frequency at which the narrower pattern begins to occur depends directly on the length of the tube. If you shorten the tube, you raise the transition frequency, which was rather high to begin with. Also, the narrowness of the pickup pattern at high frequencies will be less. So if you shorten the tubes, you wouldn't be reducing the "shotgun effect" of your microphones by a constant amount across the spectrum; instead you'd be reducing the effect, but also limiting it to frequencies above where most of the acoustic energy of music is. To put it bluntly, this would make the shotgun aspect of the microphones almost completely valueless.
The whole physical principle of the interference tube depends on sound wavelengths; the design can't be miniaturized and still be effective, as long as the medium of sound transmission is air.
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