The concussion of a tank's main gun muzzle blast can indeed shatter helmet liners and the old solid one-piece tankers CVC helmet; I've done that myself when outside the vehicle and too close to the muzzle [the idea for the guy on ground security with a .45 *Greasegun* was to stay where the driver could see him, and thereby not get run over] with an M48A3's 90mm gun and around M60A1s on gunnery ranges; the blast from the short tubed M551 Sheridan was even worse. And you most certainly do not want to be in the vicinity of forward of the muzzle of a tank's main gun when discarding sabot or canister rounds let go.
As to the possibility of a close one from a .50, I'm aware of a crew that let fly with the .50 in the commander's cupe at a fella at around 75 meters and missed him, [no boresight on the commander's optics suspected] though he was unconscious when the crew dismounted to collect his weapons and documents for S2 as well as any souveniers available. To their surprised he was untouched and alive but unhit. Whether that was from concussion from the .50 rounds or shock and panic as .50 tracers went flying by his face this deponent knoweth and sayeth not. But I would not care to try being on the receiving end to find out.
-archy-/-