Lead bullets typically feature one or more grooves in the rearward “straight” portion of the projectile, which will be filled with bullet lube. What is bullet lube, what does it do, and how does it do it?
“Bullet lube” is distinct from “case lube,” the latter of which is intended to coat casings and lessen the amount of force required to cycle them through the reloading machine. Without jumping down a really deep rabbit hole here, let’s just say that, in simplest terms, bullet lube is typically a concoction of wax or grease, perhaps with some additives, soft enough to fill a lube groove, solid enough to stay in place until the bullet is fired.
When we fire a bullet, barrel-to-bullet friction causes the bullet to heat to temperatures well beyond the melting point of most metals. Bare lead bullets are notorious for “leading barrels,” i.e. melted lead coats the bore (hole through the center of the barrel) which, as it builds-up, results in decreased accuracy and increased pressures.
In order to prevent this, lube is place in the aforementioned lube grooves. When the bullet heats, so too does the lube, which softens. It then exits the grooves and coats the bore. This bullet-to-bore lube transfer is caused by three mechanisms: compression, linear acceleration, and radial acceleration.
(1) Compression: When we fire a bullet, its base is suddenly hit with tens of thousands of pounds per square inch of gas pressure. This causes the bullet base to swell.
(2) Linear (inline or straight) acceleration: As the bullet is driven forward, it hits the rifling. The resistance of the rifling to the bullet, and the force of gas pressure still hitting the bullet base, causes the bullet to actually compress and swell. As the bullet swells, the lube grooves get smaller, forcing lube out of them.
(3) Radial acceleration (i.e. spin): As the bullet travels down the rifling, spin is imparted to the bullet. This circular motion of the bullet also does its bit to fling bullet lube out of the groove onto the bore.
In addition to spreading the bore with, essentially, a slick coating to prevent melted lead from sticking to the interior of the barrel, theoretically at least bullet lube can cause micro-cracks or imperfections in the bullet, and likewise imperfections in the bore, to fill-in, decreasing flame cutting (gas pressure flowing past the base of the bullet, between the bullet and bore, scoring and deforming the bullet). Therefore, again theoretically at least, bullet lube leads to increased accuracy.
Those who cast their own bullets will always have their preferred mixture of bullet lube, often arrived at through a long process of trial and error. Commercially available bullets will typically come with lube already applied.
Lead bullets have serious drawbacks: handling the bullets will swiftly coat your fingers with black lead dust, firing them produces clouds of vaporized lead smoke which can be a health concern, and the lube can smear off bullets at the bullet seating die, and to a lesser extent the crimp station, clogging the dies which must then be cleaned-out. Having said all that, bare lead bullets have always been with us, and always will be.
