Thande
Donor
My favourite has to be the planned German sonic arsenal.
Originating from the Austrain Dr. Zippermeyer whose response to the ferocious Allied air bombardment of the Reich was to experiment with both wind and sound as potential anti-aircraft weapons.
One such device was the Windkanone or ‘Whirlwind Cannon’, which was meant to produce artificial ‘whirlwinds’ by generating explosions in a combustion chamber and directing them through specially designed nozzles at the target. Experiments with a small cannon supposedly shattered planks at 200-yard (183m) range, and a full-size one was built. Fortunately for British and American aircraft, the effect was impossible to reproduce at high altitudes and the project was scrapped. The huge hulk of the ‘Whirlwind Cannon’ itself, though, was discovered rusting and abandoned by bemused Allied forces on the Artillery Proving ground at Hillersleben in April 1945.
That's interesting, because I remember reading that there were rumours in Britain about an "airblast gun" being developed which were apparently a complete hoax, but perhaps not...