Quoted:
When running courses, I specifically ask attendees not to bring a blaster with a muzzle break with the large gills, like a Miculek or JP "tank" break, because I and my safeties will be sick to our stomachs, with headaches halfway through the first day. Some guys even get vertigo from being around them, unless they're shooting the offending weapon, where the sonic lobes don't resonate. Sounds like a bunch of BS, but I have shared this phenomenon with other instructors and shooters, and we all seem to be in agreement.
It happens. In fact, don't flash-bangs work on exactly this principle? Many countries limit the number of rounds troops are allowed to fire out of large weapons, like the M/48 Carl Gustav for example, during training due to the effects of the blast wave on the body. Shoot too much, and it screws up your intestines.
To me it almost seems like while each individual shot is insignificant, some of those brakes can direct the blast in such a way that, over time, the effect becomes cumulative and noticeable. They had a "Tac-rifle" course set up at a range out here for a match once. They had a stage that was pretty cramped, like a mocked up hallway. A few of us were shooting big bore guns, classic battle rifles...8mm Mauser, 30-06, .303 British, etc. The RO and SO actually looked like they were suffering some physical effects after running everybody through that stage, and I remember actually feeling funny for a split second every time I pulled the trigger.