Bf-109s and FW190s against B17s

The 'significant event' as Henry brown described the first (and only) use was that he thought a barrage of flak erupted in front of him when he started a strafing run.

Yep, definitely get your attention and/or require a trip to the laundry
not to mention a 3:00 am meeting behind the outhouse with the guy who authorized the use of the ammo

This does look like the most promising explanation of the experiences of your father and his friends unless an unknown round makes a sudden appearance.