A full metal jacket (FMJ) is a type of bullet with a soft lead core wrapped in a harder metal shell, typically copper or a copper alloy. The jacket covers the entire bullet except for a small opening at the base. This design keeps the bullet intact as it travels through the air and into a target, rather than mushrooming or breaking apart on impact. FMJ is the most common ammunition type in the world, used by every major military and fired by millions of recreational shooters at ranges every day.
The term also refers to Stanley Kubrick’s 1987 Vietnam War film, which took its name from the ammunition. But if you’re here wondering what the phrase actually means in a physical sense, it’s all about the bullet.
How an FMJ Bullet Is Built
The “jacket” is a thin shell of harder metal that encases a softer core. The core is almost always lead, chosen because it’s dense and heavy for its size. The jacket is most commonly copper or brass, though manufacturers also use nickel, aluminum, and steel. This outer layer serves a few purposes: it prevents the soft lead from deforming inside the gun barrel, it reduces lead fouling (residue buildup) in the barrel, and it helps the bullet maintain its shape during flight.
The jacket wraps from the bullet’s tip down to the base, leaving only a small circle of exposed lead at the bottom. This distinguishes it from a “total metal jacket,” which covers the base too, and from a hollow point, which has an open cavity at the tip designed to make the bullet expand on contact.
What Happens When It Hits a Target
The defining characteristic of FMJ ammunition is that it holds together. When an FMJ bullet strikes soft tissue or a ballistic gel block (the standard lab substitute for flesh), it tends to pass straight through rather than expanding or fragmenting. In ballistic testing, FMJ rounds consistently over-penetrate, meaning they exit the far side of the target with enough energy to keep going. One study comparing bullet types found that the FMJ had to be excluded after just three experiments because it punched completely through every test block, including through bone samples on both ends, something the expanding bullet designs did not do.
This doesn’t mean FMJ rounds create a clean, narrow hole. As the bullet decelerates inside tissue, it can yaw, which means it tumbles end over end. A 9mm FMJ fired from a pistol, for example, is well documented to tumble through ballistic gelatin, creating a temporary cavity as tissue is pushed outward before snapping back. The permanent wound track left behind is narrower than what an expanding bullet produces, but the deep, straight penetration is what makes FMJ rounds distinctive and, in certain contexts, dangerous.
Expanding bullets like hollow points work on the opposite principle. They’re engineered to mushroom open on impact, dumping their energy quickly and creating a wider wound channel. That rapid energy transfer means they slow down fast and are far less likely to exit the target. In the same ballistic study, hollow point designs fragmented heavily and stopped inside the test medium, while the FMJ sailed through every time.
Why Militaries Use FMJ
FMJ became the global standard for military ammunition because of a treaty signed in 1899. The Hague Declaration (IV, 3) prohibited the use of bullets that “expand or flatten easily in the human body,” which effectively banned hollow points and soft-point rounds in warfare. The reasoning was rooted in a principle that still underpins international humanitarian law: weapons should disable enemy combatants, not cause suffering beyond what’s necessary to take someone out of the fight. Expanding bullets were seen as inflicting unnecessarily severe wounds compared to FMJ rounds, which were considered more likely to incapacitate without maximizing tissue destruction.
The declaration entered into force in September 1900 and has shaped military ammunition design ever since. The standard NATO rifle round, for instance, is a 5.56x45mm FMJ weighing 62 grains with a copper jacket over a lead core and a steel penetrator tip. It leaves the barrel at roughly 3,110 feet per second. The older U.S. M193 variant is lighter at 55 grains and slightly faster at about 3,250 feet per second. Both are full metal jacket designs built to meet international legal standards.
Why It’s Popular at Shooting Ranges
For civilian shooters, FMJ is the default choice for target practice and training, and cost is the biggest reason. The design is simple to manufacture compared to hollow points or specialty hunting rounds, which keeps the price per round significantly lower. If you’re putting hundreds of rounds downrange in a single session, that difference adds up fast.
FMJ also feeds reliably in semi-automatic firearms. The smooth, rounded profile of the bullet slides cleanly from the magazine into the chamber, reducing the chance of jams or feeding failures. This makes it a go-to for breaking in new guns or testing firearm function. The structural integrity of the metal jacket also means the bullet is less likely to deform during the loading and firing cycle, which contributes to consistent accuracy shot after shot.
The Over-Penetration Problem
The same quality that makes FMJ effective for military use and reliable for range shooting creates a serious liability for self-defense. Because the bullet doesn’t expand or fragment, it retains enough velocity after passing through a person’s body to injure or kill someone standing behind them. FMJ rounds in common handgun calibers (9mm, .40, .45) are all highly likely to over-penetrate a human torso with lethal force remaining on the other side. They can also punch through interior walls, car doors, and other barriers that might separate a threat from bystanders.
This is why most self-defense and law enforcement ammunition is hollow point rather than FMJ. A hollow point that expands properly dumps its energy into the first target and stops. An FMJ keeps going. The risk is compounded in real-world situations where a shooter may not be able to see what’s behind the person they’re firing at, whether because of darkness, tunnel vision under stress, or simply because the target’s body blocks the view.
There’s a secondary concern worth knowing: if a hollow point bullet gets plugged by heavy clothing or other material on the way in, it can fail to expand and behave like an FMJ round, penetrating much deeper than intended. This is one reason ammunition manufacturers continually refine hollow point designs to ensure reliable expansion through barriers.
FMJ vs. Hollow Point at a Glance
- FMJ: Holds its shape on impact, penetrates deeply, tends to pass through soft targets, creates a narrower wound channel, cheaper to manufacture, standard for military use and range training.
- Hollow point: Expands on impact, transfers energy quickly, stops inside the target, creates a wider wound channel, costs more per round, preferred for self-defense and law enforcement.
The choice between the two comes down to purpose. FMJ is built to be affordable, reliable, and compliant with international law. Hollow points are built to stop a threat without continuing through it. Neither is universally “better.” They’re engineered for different jobs.