Bullet, you know the real thing, I assume - mine were exercising granades. I'll never forget the sound they did, when they passed us very closely, short before impact. I was on a 14 meter wooden watchtower, and it sounded, as if some giant had thrown complete washing machines through the air - a vibrating, "spiraling hiss and hum" sort of sound.
Down again, the officer told me, that only 3 weeks ago, a granade had come too short and slamed into an Unimog army van (luckily no one in there).
My knees began to feel rather wobbly - now he told me! (Lol!)
PS: but we shouldn't go on talking army here, I suppose...