A bullet traveling down a barrel at 3,000 fps (about 915 m/s) is subject to extreme friction and heat. This friction not only wears the barrel but also causes a microscopic layer of the bullet’s copper jacket to be "smeared" into the rifling. This copper build-up eventually degrades accuracy. The goal of bullet coating is to introduce a barrier of lubricity between the jacket and the bore.

Molybdenum Disulfide (Moly)

Molybdenum disulfide (MoS₂) was the first mainstream bullet coating. It is a solid lubricant—the same compound used in high-performance automotive and aerospace applications—that, when applied to a bullet’s surface, reduces the coefficient of friction significantly. Application is typically performed by “impact plating”: tumbling bullets with small steel balls and moly powder in a sealed container for one to two hours. The steel shot mechanically drives the moly into the microscopic pores of the copper jacket.

Early adopters in the 1990s reported reduced copper fouling and higher velocities at equal chamber pressure. That last point is often misquoted. At an equal powder charge, the coating lowers both velocity and pressure: Vaughn measured 3,175 fps and 54,000 psi uncoated against 3,083 fps and 47,000 psi coated, on the same 68-grain 6 mm bullet—2.9 % less velocity for 7,000 psi less pressure. The velocity gain comes from raising the charge back to the original pressure. The same test, alternating coated and uncoated bullets, showed no residual effect of the coating on subsequent shots.

That raised charge is the trap. It is worked up again from the starting load, like any new load; and it belongs to the coated bullet alone. Fired behind the same bullet uncoated, it loses the margin the coating provided (7,000 psi at equal charge in the test above) and lands above the pressure it was developed to. Coated and uncoated bullets therefore get separate load development and are never mixed on the bench.

The practical drawback is the pervasive black residue, which coats the reloading room and requires specialized solvents. A moly-fouled bore left uncleaned is widely held to corrode; the mechanism offered varies between sources, and we have no measurement to set against it—the usual caution (clean and dry after a session) remains sensible, but we do not present it as established fact.

Hexagonal Boron Nitride (hBN)

Hexagonal boron nitride, commonly known as “white moly,” has largely superseded molybdenum disulfide in the competition world. hBN is chemically inert and does not attract moisture, eliminating the corrosion concerns that plagued moly. It is a white powder that produces far less mess during application and does not leave a persistent residue on equipment. Its lubricity is comparable to or better than moly’s, providing a consistent, dry film that is extremely effective at reducing copper jacket smear in the bore. For the reloader who chooses to coat, hBN represents the superior option by virtually every practical measure.

For the DIY reloader, impact plating remains the most effective way to apply these coatings.

1. Aggressive Cleaning: Bullets must be surgically clean and degreased. Any oil or residue from the manufacturing process will prevent the coating from adhering. Warming the bullets to \(120^\circ\)F–\(140^\circ\)F helps the plating process.

2. Tumbling: The bullets are tumbled with steel shot and the lubricant powder. The steel shot "peens" the lubricant into the surface of the copper jacket.

3. The Wax Seal (Optional): Some reloaders follow the coating process with a brief tumble in carnauba wax. This "seals" the coating, making the bullets less messy to handle and providing an extra layer of protection against the elements.

The most significant modern evolution in the battle against fouling is the introduction of chemical additives directly into the propellant.

Copper Fouling Eraser (CFE)

Pioneered by Hodgdon and IMR, CFE technology involves the inclusion of a proprietary chemical agent in the powder that physically reacts with copper during the combustion process. CFE stands for Copper Fouling Eraser. The manufacturer claims a strong reduction in copper deposition but does not publish the mechanism, which it holds as proprietary. It is often written that the additive prevents copper from metallurgically bonding to the barrel steel; that explanation is plausible and widespread, but no published source establishes it. What is documented is the effect, not its cause. For the precision shooter, CFE powders (such as CFE 223 for rifle applications) offer the benefits of reduced fouling without the labor-intensive process of impact plating bullets with hBN or moly. This allows for significantly longer shooting strings between cleanings, maintaining accuracy for dozens or even hundreds of rounds. The practical result is less time at the cleaning bench and more confidence that the rifle’s accuracy will not degrade during a long match or a high-volume practice session.

The choice of bullet and its match to the barrel’s twist rate affect both accuracy and fouling; this section covers them before the conclusions on coatings.

The Ogive and Ballistic Coefficient

The ogive—the curved forward portion of the bullet ahead of the bearing surface—is the primary determinant of aerodynamic drag. Modern precision bullets fall into several ogive categories: In a tangent ogive design, the curve blends smoothly into the bearing surface at a tangent, creating a gentle transition. Bullets with tangent ogives—such as the Sierra MatchKing—are generally less sensitive to seating depth and perform well across a wide range of chamber dimensions. Their ballistic coefficient is typically moderate to high, and their forgiving nature makes them an excellent choice for reloaders who value consistency over maximum aerodynamic efficiency.

The secant ogive takes a more aggressive approach: the curve intersects the bearing surface at an angle, creating a sharper, more streamlined profile. VLD (Very Low Drag) bullets from Berger use this design to achieve very high ballistic coefficients, but the sharper geometry makes them more sensitive to seating depth and they may require more careful load development to achieve their full potential.

The hybrid ogive, pioneered by Berger, represents a modern compromise that has become the dominant choice in precision rifle competition. It combines a tangent section near the bearing surface—which provides the seating-depth tolerance that makes tuning easier—with a secant section forward that delivers the aerodynamic efficiency needed for long-range performance. The result is a bullet that shoots nearly as well as a pure secant design at distance, while being far easier to tune in a variety of rifles and chambers.

Twist Rate and the Stability Factor

The barrel’s twist rate must spin the bullet fast enough to achieve gyroscopic stability. An under-stabilized bullet will “keyhole” (strike the target sideways).

It is often written that a marginally stable bullet flies well up close but loses stability as it slows through the transonic zone. Gyroscopic stability does not work that way: it rises downrange, and keeps rising through transonic. Spin decays far more slowly than velocity, and \(S_g\) scales as \(p^2/v^2\). Integrating the .308 Winchester 168 gr of Chapter 5’s worked example with measured spark-range coefficients, \(S_g\) climbs from 1.7 at the muzzle to 6.2 at 1,100 yards—the spin has lost 20 % while the velocity has lost 63 %, and the 29 % rise in the overturning-moment coefficient across transonic comes nowhere near offsetting it. McCoy draws the same conclusion from the same bullet: from a 12-inch twist at 2,600 fps, \(S_g\) rises from 1.7 at the muzzle to 5.8 at 1,000 yards (McCoy 2012, 198), where the same integration gives 5.85.

What genuinely degrades at transonic is dynamic stability—the ability to damp a disturbance rather than the ability to resist overturning. The two are different criteria, and \(S_g\) is a necessary condition, not a sufficient one. McCoy describes the growing yaw of a bullet whose \(S_g\) already exceeds 2 as “not a gyroscopic instability” but a “small-yaw dynamic instability” (McCoy 2012, 198), and notes that the effect of a boattail on dynamic stability “at transonic and subsonic speeds is generally negative” (McCoy 2012, 84). A bullet marginal at the muzzle is therefore a problem at the muzzle, where its \(S_g\) is lowest: that is where cold, dense air can push it under the threshold.

The Miller Stability Factor (\(S_g\)) is the modern method for evaluating bullet stability. An \(S_g\) of 1.5 or higher is considered fully stable under all conditions. Values between 1.3 and 1.5 indicate marginal stability that may become problematic in cold, dense air (which increases aerodynamic forces on the bullet). The formula takes into account bullet length, weight, diameter, and the barrel’s twist rate, as well as air density. Bryan Litz’s Applied Ballistics software provides an excellent calculator for determining \(S_g\) for any bullet-twist combination.

As a general rule, longer and heavier bullets require faster twist rates. The trend in modern precision cartridges toward long, heavy, high-BC projectiles has driven barrel makers to offer faster twists than were common a generation ago. For example, the .308 Winchester has moved from the traditional 1:12” twist to 1:10” and even 1:8” to stabilize modern 200+ grain bullets designed for extreme long-range shooting.

Today, the use of bullet coatings is no longer considered a universal requirement for accuracy, and the decision to coat depends on the specific shooting discipline and round count. Coating remains highly valued by varmint hunters and high-volume competitors who may fire hundreds of rounds in a single session without the opportunity to clean their rifles; the reduction in copper fouling allows the rifle to stay in its “accuracy window” for much longer strings.

However, many top-tier benchrest and F-Class shooters have moved away from DIY coatings in favor of premium, uncoated bullets from Berger or Sierra, finding that the modern quality of match-grade barrels and precision bullet jackets has reduced fouling to a level where the marginal benefit of coating does not justify the additional process step. For these shooters, the consistency of uncoated, factory-inspected bullets is more valuable than the friction reduction offered by hBN or moly.

A middle ground exists in the form of factory-applied coatings. Manufacturers like Winchester utilize Lubalox (a black oxide coating), which provides a durable layer of lubricity without the mess and inconsistency of home-applied treatments. Similarly, some boutique bullet makers now offer hBN-coated bullets as a standard option, allowing the shooter to benefit from coating technology without investing in the equipment and process time required for DIY application.

References