Forensic investigators determine which bullet struck glass first by examining where the crack lines stop. When a second bullet hits glass that already has fractures from a first impact, the new cracks run outward until they meet the existing fracture lines, then terminate. This principle, known as fracture termination or the “3R rule,” is one of the most reliable tools in forensic glass analysis.
How Glass Breaks From a Bullet Impact
When a bullet strikes a pane of glass, it produces two distinct types of fracture lines. Radial fractures shoot outward from the point of impact like the spokes of a wheel. Concentric fractures form roughly circular rings around the impact point, connecting those radial lines. Together, they create the classic spiderweb pattern you see around a bullet hole.
These fractures don’t just sit on the surface. They extend through the full thickness of the glass, creating a permanent structural change in the pane. That structural change is what makes sequencing possible, because the next bullet’s fractures interact with the ones already there.
The Fracture Termination Principle
Glass can only crack along a path where it’s still intact. Once a fracture line already exists, it acts like a barrier. When a second bullet strikes the same pane, its radial and concentric fractures spread outward normally until they reach a pre-existing crack. At that point, the new fracture stops dead. It has nowhere to go because the glass is already broken along that line.
This gives investigators a clear rule: the fracture lines that stop at another set of fracture lines came second. The fractures that run uninterrupted came first. By tracing which cracks terminate against which, analysts can reconstruct the order of multiple impacts. If bullet hole A’s radial fractures extend freely across the glass while bullet hole B’s radial fractures terminate when they reach A’s cracks, bullet A hit first.
This works with any number of impacts, not just two. Each successive bullet adds fractures that terminate against all previously existing ones, allowing investigators to build a complete sequence from first shot to last.
Identifying Entrance vs. Exit Side
Sequencing tells you the order, but investigators also need to know which side of the glass the bullet entered from. Glass fractures create a cone-shaped pattern called cratering, where the hole is smaller on the side the bullet entered and wider on the side it exited. If you look at the bullet hole in cross-section, the glass flares outward in the direction of travel.
On the exit side, you’ll typically see more chipping and missing glass around the edges of the hole. The entrance side tends to be cleaner and more defined. This cone shape is visible even in small fragments if enough of the hole’s edge remains intact.
Reading the Rib Marks on Fractures
For additional confirmation, examiners look at the broken edges of glass along fracture lines. When glass snaps, it doesn’t break cleanly. The broken edge shows curved ridges called rib marks, sometimes also called hackle marks or Wallner lines. These tiny curved lines on the fractured edge reveal information about the direction of force.
On radial fractures, the rib marks curve so that their concave side points toward the side the force came from (the side the bullet entered). On concentric fractures, the pattern reverses. This relationship, sometimes called the “3R rule” (radial cracks, right angles, rear side), gives investigators a way to confirm the direction of impact even when they’re working with glass fragments rather than an intact pane.
When Sequencing Gets Difficult
The fracture termination method works best when the two bullet holes are close enough that their fracture lines actually meet and interact. If two shots hit opposite ends of a large window, their radial cracks may never reach each other, leaving no termination points to analyze. In that case, sequencing from fracture patterns alone may not be possible.
The type and thickness of glass also matters. Tempered glass, the kind used in car side windows, shatters into small granular pieces rather than producing the radial and concentric pattern needed for this analysis. Laminated glass, like car windshields, holds together better and can retain useful fracture information, but the plastic interlayer complicates the pattern. Standard plate glass and float glass used in most building windows give the clearest results.
Environmental factors can further complicate things. If the glass partially collapses after the first shot, pieces may be missing by the time the second bullet arrives. Heat, existing stress in the glass, and the angle of impact all influence how fractures propagate. Investigators document everything photographically before touching the scene, since even small vibrations can cause additional fracturing in already-weakened glass.
How Investigators Examine the Scene
Analysts typically examine bullet-perforated glass in place whenever possible, photographing it from both sides with oblique lighting to highlight fracture lines. They map each fracture, noting which lines are continuous and which terminate against others. The glass around each bullet hole is examined with magnification to identify rib marks and cratering direction.
If the glass has fallen or shattered, fragments are collected and reassembled like a puzzle. Even partial reconstruction can reveal enough termination points to establish sequence. Forensic labs may use specialized lighting and microscopy to examine rib marks on individual fracture surfaces, confirming the direction of force for each impact.
The analysis produces a sequence (first, second, third) but cannot determine the time gap between shots. Two bullets fired a half-second apart and two bullets fired an hour apart produce the same fracture termination patterns. Timing information has to come from other evidence, like witness accounts or audio recordings.