A hollow point bullet has a cavity machined into its tip that causes the bullet to mushroom open on impact, expanding to roughly 1.5 to 2 times its original diameter. This expansion serves two purposes: it transfers more energy into the target, and it slows the bullet down so it’s less likely to pass through and strike something behind it. The design is simple, but the physics behind it are worth understanding.
What Happens on Impact
When a hollow point strikes a fluid-dense target, liquid is forced into the open cavity at the nose. That sudden pressure pushes outward against the thin walls of the bullet, peeling them back like the petals of a flower. This all happens in milliseconds. The expanded bullet, now significantly wider than it started, plows through tissue creating a larger wound channel while rapidly decelerating.
A full metal jacket round, by contrast, keeps its shape. It punches a narrow, bullet-diameter hole and often retains enough velocity to exit the other side. That pass-through means less energy is deposited in the target and a greater risk that the bullet continues traveling into whatever or whoever is behind it.
Energy Transfer and Wound Channels
A bullet in motion carries kinetic energy. What matters in terminal performance is how much of that energy gets transferred to the target versus how much the bullet carries out the other side. A hollow point that expands and stays inside the target deposits all of its remaining kinetic energy. A bullet that exits deposits only a fraction.
Two types of wound channels form when a bullet enters tissue. The permanent cavity is the hole left behind, proportional to the bullet’s size. Since a hollow point expands, its permanent cavity is significantly wider than that of a non-expanding round. The temporary cavity is a brief, violent stretching of surrounding tissue caused by the energy transfer. This temporary expansion can reach up to 30 times the bullet’s diameter and lasts only 5 to 10 milliseconds, with internal pressures of 100 to 200 atmospheres. The combination of a wider permanent channel and a larger temporary cavity is what makes hollow points more effective at stopping a threat quickly.
If a bullet tumbles sideways during its path (a phenomenon called yaw), the tissue damage can be more than three times as extensive as a bullet traveling nose-forward. Hollow points that fragment on impact create additional secondary projectiles, with pieces weighing less than a gram still capable of penetrating 10 to 15 centimeters of soft tissue on their own.
How the Bullet Is Engineered to Expand
The hollow cavity alone isn’t the whole story. Manufacturers use several design features to control how the bullet opens up. One of the most common is skiving: small pre-cut notches scored into the bullet’s copper jacket around the cavity opening. These notches act like perforated lines on a sheet of paper, guiding where the jacket splits so the petals peel back symmetrically. Without skiving, expansion can be lopsided, sending the bullet off course.
The jacket thickness, cavity depth, and lead core hardness all influence how much the bullet expands and how deep it penetrates before stopping. A deeper cavity with thinner walls expands more aggressively but may not hold together. A shallower cavity with thicker walls penetrates deeper but expands less. Ammunition designers balance these variables for different intended uses.
Polymer-Tipped Designs
One persistent weakness of traditional hollow points is that the open cavity can get clogged. Heavy clothing, drywall, or other barrier materials can pack into the hollow tip, effectively turning the bullet into a round-nose projectile that passes straight through without expanding. This is a real-world problem, not a theoretical one, and it’s the reason the FBI tests ammunition through layered fabric, auto glass, and sheet metal before approving it for duty use.
To solve this, some manufacturers fill the cavity with a polymer insert. The bullet looks and flies like a conventional round-nose projectile, improving aerodynamic consistency. On impact, the polymer plug is driven backward into the lead core, forcing the jacket outward from the inside rather than relying on fluid pressure alone. This mechanical expansion works even when the cavity would otherwise be plugged with debris. Federal’s Expanding Full Metal Jacket was one of the first designs to move away from fluid-dependent expansion entirely, using an internal polymer pill to initiate mushrooming through compression.
The FBI Testing Standard
The most widely referenced performance benchmark for hollow point ammunition comes from the FBI’s ballistic gelatin protocol. Calibrated 10% gelatin simulates the density of human soft tissue, and the FBI requires that a bullet penetrate between 12 and 18 inches of it to be considered adequate. Anything less than 12 inches risks failing to reach vital structures. Anything beyond 18 inches suggests the bullet is over-penetrating.
The protocol includes six separate scenarios: bare gelatin, heavy clothing over gelatin, and shots through intermediate barriers like auto glass, plywood, sheet metal, and wallboard. A hollow point has to expand reliably across all six to pass. For handgun ammunition, the expansion requirement is at least a 50% increase in diameter. For rifle ammunition, it’s 100%. So a 9mm handgun bullet starting at .355 inches needs to open up to at least .53 inches, while a .308 rifle bullet starting at .308 inches needs to reach .616 inches or wider.
Penetration is weighted more heavily than expansion in the FBI’s evaluation. A bullet that penetrates to the right depth but doesn’t expand much is considered more acceptable than one that expands dramatically but stops too shallow.
Why Hollow Points Are Banned in War but Used by Police
Hollow point ammunition is prohibited in international armed conflict under the 1899 Hague Declaration, which specifically targeted expanding bullets (then called “dum-dum” rounds after the British arsenal in India where they were first produced). The legal reasoning centers on the principle that weapons causing unnecessary suffering in warfare are prohibited. Expanding bullets were judged to exceed the level of injury needed to take a combatant out of the fight.
The United States never formally ratified the Hague Declaration but generally abides by it in military operations, interpreting the restriction through the lens of the 1907 Hague Regulations’ broader prohibition on weapons causing unnecessary suffering.
Law enforcement operates under a completely different legal framework. Police aren’t bound by the laws of armed conflict, and the priorities are reversed. In a civilian environment surrounded by bystanders, walls, and vehicles, over-penetration is the primary safety concern. A full metal jacket round that passes through a threat and into a crowd creates exactly the kind of collateral risk that hollow points are designed to prevent. That’s why virtually every major law enforcement agency in the United States issues hollow point ammunition as standard duty ammo. The reduced penetration and faster energy transfer make it the safer choice in populated settings.
Why Expansion Sometimes Fails
Hollow points are not guaranteed to expand every time. The most common cause of failure is cavity clogging. Layers of heavy winter clothing, particularly denim and insulation, can pack into the hollow tip tightly enough to prevent the walls from opening. When that happens, the bullet behaves like a solid round: it punches through with minimal expansion, penetrates deeper than intended, and may exit.
Auto glass presents a different problem. Angled windshield glass can deflect a bullet off its intended path and strip away jacket material before the round even reaches its target. Sheet metal, like a car door, can have a similar effect. These are the exact scenarios the FBI protocol is designed to test, and they’re the reason modern premium hollow points have become increasingly engineered. Designs like the Federal HST use jacket geometry that resists clogging even through heavy barriers, though no design is completely immune.
Velocity also matters. A hollow point needs a minimum impact speed to generate enough hydraulic pressure for expansion. If the bullet has traveled a long distance and slowed significantly, or if it passes through a barrier that absorbs much of its energy, it may not have enough velocity left to open up. This is why ammunition is typically rated for specific barrel lengths and effective distances.