The Bullet Hole Masquerading as a Rock

A window with a neat circular hole punched through it, edges smooth, no spiderweb of cracks radiating outward — that is not the signature of a thrown rock. It's the signature of something moving very fast and very small. A pellet, a BB, or a bullet.

The physics comes down to a competition between two speeds: how fast the projectile arrives, and how fast a crack can travel through glass. Cracks in ordinary annealed glass propagate at roughly 1,500 metres per second. A rock lobbed by an eight-year-old is doing maybe 15 metres per second when it lands. At that leisurely pace, the glass has ample time to flex, transfer the energy across its whole surface, and then fail everywhere at once — the shatter we all recognise.

A projectile travelling faster than the crack can spread changes the whole story. The glass immediately in front of the impact simply gets knocked out before the surrounding material has any idea what happened. The energy stays local. You get a clean punch-out, sometimes with a shallow cone chipped from the far side, and a pane that is otherwise intact. Ballistics investigators read these holes like handwriting: the cone always widens in the direction the projectile was travelling, so the wide end tells you which way it was going.

This is the same principle behind an old party trick — firing a bullet through a hanging sheet of paper leaves a hole rather than tearing the whole thing off its clip, provided the round is fast enough. It's also why you can sometimes flick a playing card through a watermelon's skin, though that one is more about a sharp edge than sheer velocity.

The unsettling implication, of course, is that a hole like this in a domestic window rarely comes from a well-aimed pebble. Air rifles produce exactly this pattern. So do the small-calibre firearms that people occasionally discharge with more enthusiasm than aim. The tidy circle that looks like a curiosity is, in forensic terms, a flag.

The practical response is less dramatic. A hole that clean, in a single pane, patches easily enough — a disc of clear tape, a dab of clear silicone, or, for the fatalist, a tennis ball wedged into the gap until the glazier arrives. Whether you should also be having a word with the neighbours depends entirely on what they keep in the garage.