Vortex weapons

I have heard of Nazi produced Vortex Weapons, which are like cannons that sent out sonic-boom-like blasts. They can snap a board like a twig, but when fired at a human, felt like being hit by a mattress. How does this work? How can they create a sonic boom in a cannon barrel? And why does it blast you back?

      • There’s vortex pages all over the web, not so far I haven’t seen anything powerful enough to be considered a weapon against anything besides card houses. —One starting point: http://www.amasci.com/amateur/vortgen.html
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