Busted...
Complete Myth... one of my favorite though.
If a shock wave existed for a .50 Caliber rifle round it would exist for all projectiles of similar size and muzzle velocity. Fact, a 30-06 round has a diameter difference of only .2" inches yet has the same approximate muzzle velocity, yet no one is afraid of near miss from a dear rifle. A bullet penetrates and object and does horrific damage as it begins to tumble through your body as it penetrates flesh, bone, and muscle. Hence the small entrance wound and typically bigger exit wound.
Next time you have the opportunity to closely inspect a target at a gun range, you will notice the bullets leave holes the size of the bullet passing through the paper... If your shock wave existed and could cause significant damage, then one near miss shot would tear the paper.
True, a 50 caliber round does have a muzzle velocity more than speed of sound but trust the Engineer, .50 cal, like all other firearms, require you to strike your target with the round. Near misses, are still misses... you may scare a person once the realize what just wizzed by them... but thats all.