A bullet is the projectile component of a cartridge, typically composed of a metal core launched at high velocity toward a target. The vast majority of these projectiles are made from lead, often encased in a copper or brass alloy jacket. This material choice might seem counterintuitive, as steel is widely known for its superior strength and hardness in other applications. Why steel is generally avoided for standard bullet construction relates to the unique requirements of propelling a projectile down a rifle barrel and its subsequent interaction with air and a target.
The Advantages of Lead and Copper
The selection of lead as the primary bullet material is due to its exceptional physical properties that optimize performance. Lead possesses a high density, approximately 11.3 grams per cubic centimeter, which is significantly greater than steel’s density of about 7.8 grams per cubic centimeter. This high mass-to-volume ratio allows a lead projectile to maintain its momentum and resist air resistance more effectively as it travels downrange, a property reflected in a better ballistic coefficient. A projectile with greater momentum retains velocity and delivers more kinetic energy at longer distances.
The malleability of lead is another factor that makes it highly suitable for ammunition manufacturing. Lead is soft and easily deformed, facilitating the swaging or casting processes used to form the bullet. When fired, the soft material readily engages with the barrel’s rifling—the spiral grooves that impart spin to stabilize the projectile in flight. This deformation also helps to seal the bore, maximizing the pressure generated by the burning propellant and preventing gas from escaping past the projectile. Copper, when used as a jacket, provides a relatively soft outer layer that is harder than lead but still soft enough to grip the rifling, reduce lead fouling, and allow for controlled expansion.
Steel’s Damage to Firearm Barrels
The main reason steel is not used for standard bullet cores or jackets is the potential for accelerated mechanical wear on the firearm itself. Hard steel is often comparable to, or even harder than, the steel alloys used to construct the lands and grooves of a firearm barrel’s rifling. When a hard steel projectile is forced through a rifled barrel at high velocity, it acts as an abrasive, grinding against the barrel’s internal structure. This direct steel-on-steel contact rapidly degrades the rifling, which is the mechanism that stabilizes the bullet.
The spiraling lands of the rifling become rounded or worn down much faster than with a softer copper-jacketed lead bullet. This degradation leads to a loss of the projectile’s rotational stability and a significant decrease in accuracy over a much shorter barrel lifespan. Furthermore, steel generates a higher degree of friction compared to the softer copper and lead materials, which increases heat within the barrel. Elevated friction and heat can cause thermal stress, particularly at the throat of the barrel where the bullet first engages the rifling. The hardness of steel also means it does not deform easily to precisely fit the bore, creating a risk of pressure spikes or improper engagement with the rifling.
Poor Terminal Performance and Safety Risks
A steel projectile’s hardness, while seemingly a positive trait, translates into poor performance when the bullet strikes a soft target. Standard bullets are designed to deform or “mushroom” upon impact, rapidly increasing their frontal surface area to transfer kinetic energy efficiently into the target. Because steel is far too rigid, a solid steel projectile tends to maintain its shape and punch a small, through-and-through hole, resulting in less energy transfer and reduced incapacitation. This lack of deformation is a significant disadvantage in hunting and self-defense applications where energy dump is a primary goal.
The extreme hardness of steel also introduces a substantial safety hazard in many shooting environments. When a steel bullet strikes a hard surface, such as rock, concrete, or metal targets, it is much more likely to ricochet at an unpredictable and dangerous angle than a softer lead or copper projectile. Softer materials deform and fragment upon hitting a hard surface, absorbing energy and reducing the risk of a projectile bouncing back or deviating wildly. Since steel is less dense than lead, a steel projectile of the same physical size will be lighter, carrying less momentum and being more susceptible to deflection by wind and external factors.
Specialized Ammunition Using Steel Components
While solid steel bullets are impractical for general use, steel is integrated into specific ammunition types where its unique properties are beneficial. Some manufacturers use steel to form the jacket of a projectile, often as a cost-saving measure compared to traditional copper. These “bi-metal” jackets are typically thinly coated with copper or a polymer to minimize the abrasive steel-on-steel contact with the barrel’s rifling. However, even with a coating, these steel-jacketed bullets can still accelerate barrel wear compared to pure copper-jacketed projectiles.
Steel is also an essential component in armor-piercing (AP) ammunition, which is designed for barrier penetration rather than energy transfer in a soft target. These specialized rounds feature a core made of hardened steel, tungsten, or other very hard alloys, which is often encased within a softer jacket. The hard core is intended to defeat armor plating or other hardened barriers, accepting the trade-off of increased barrel wear and reduced terminal performance in soft targets for the sake of penetration.