The vast majority of bullets are constructed of a solid lead core with a copper covering (called a “jacket”) that contains the lead. Lead is used because it’s an extremely dense, yet cheap metal, making it perfect for giving projectiles their weight while keeping the overall size small. Copper is used because it’s strong enough to keep the softer lead from de-forming, but soft enough to allow the gun’s rifling to “grip” the bullet.
The copper jacket starts out as a cup, having been cut from a long sheet. Through a process called “drawing,” the cup is lengthened and shaped to fit the profile of the projectile they’re making. These cups will eventually end up as jackets for 5.56 NATO rounds.
However, while this process is very efficient, it’s extremely difficult to actually get the metal to encase the entire lead core without any gaps. To keep production costs down, bullet manufacturers usually leave one end of the projectile open. Which end is open – and how that’s done – determines the classification of the projectile.
Here are three 150 grain .308 caliber bullets (a “grain” is a unit of bullet weight). And while they look very similar, the way in which they were manufactured is very different. The first projectile on the left is a “full metal jacket” (FMJ) round with a solid copper point. The other two are an open tip and a soft point bullet respectively, with openings in the copper at the tips of the projectiles.