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Bugeye's slug may have dropped below the sound barrier, and didn't have the familiar crack. The rifle bullet as it flies by, continues breaking the sound barrier as goes, until it drops below 1100 fps.
The shock wave is more or less like a cone shape, and the sound doesn't reach our ear until the bullet is past, thus the old soldier's phrase.."you don't hear the bullet that gets you"! Makes sense, since the one you hear is long past.
A case where a cop is involved in a shootout with handguns on both sides, he may not hear the crack, since most handguns don't break the sound barrier, even to start with. Of course, at close range it would likely be difficult to sort trhe initial sound at the gun barrel..from any subsequent crack.
Here's a little chart i found that explains the cone shape shock wave;
https://www.researchgate.net/figure/Supersonic-bullet-shock-wave-description_fig1_224679640 Have you noticed that when a jet plane passes over, if you want to see the plane, you should look some distance ahead of where the engine noise seems to be coming from ?
I'll try to find an engagement on video which illustrates the sound.