The introduction of smokeless powder presented challenges to weapons manufacturers. Because nitrocellulose-based propellants produced higher temperatures and pressures than black powder, they moved bullets down the barrel with greater velocity. As they made the journey, softer lead bullets couldn't stand up to the increased friction. Their outer layers were stripped off and left in the barrel, causing fouling.
The solution, of course, was to give bullets a thicker skin, or a jacket. Gunmakers chose copper or alloys of copper and zinc to cover their pistol bullets. They used a harder jacket of steel or cupronickel for rifle and machine-gun bullets. In both cases, the core of the bullet still contained lead, except in armor-piercing bullets, which used inner cores of hardened steel.
In military weapons, bullets possess a full-metal jacket (FMJ), meaning the jacket covers the entire projectile. These bullets are sometimes called non-expanding because they retain their shape as they pass through a target. For soldiers and military surgeons, this is a good thing, for FMJ bullets do less damage to internal tissues and organs. Big-game hunters have far different requirements. They need a bullet that will cause massive internal trauma so their prey will go down quickly. They use expanding bullets, which mushroom out as soon as they encounter resistance. The jacket of such a bullet only extends over a portion of the lead projectile, leaving the tip exposed. When a soft-point bullet strikes a target, such as a deer or a bear, the tip expands and flares out, allowing it to inflict more damage on internal organs.