Bore erosion. Bullets and other projectiles depend on a tight gas seal in the bore of the weapon. Lead used to be the solution. Lead hollow based projectiles in rifles would expand and tightly seal the barrel. However soft lead bullets do not have much in the way of armor penetration.
When high power rifles came into being, advances in bullet design and shape dictated that bullets be jacketed, to reduce lead fouling of the rifling of the weapon. Iron is too brittle and too hard to make a good projectile.
Bullets ended up with a lead or steel core surrounded by a softer outer jacket to prevent fouling or erosion by the harder steel.
By erosion, the lands of the rifling can be worn down over time by firing a lot of rounds through the barrel. So using softer copper or brass as a jacket reduced this and reduced lead fouling.
Today a lot of military bullets have a steel penetrator core or a steel alloy to increase mass to maintain acceptable inertia in the bullet.
Today the US Army is toying with environmentally friendly bullets that will biodegrade after firing. I guess they ate hoping for the Sierra Club and Green Peace seals of approval. I wonder how much these eco friendly bullets will degrade effectiveness down range, and how many US service men’s lives it will cost us in the next war. You can be sure the enemy won’t be firing eco friendly bullets, but good old lead and steal killers.
Traditional sporting and military bullets were lead and pressed into copper or brass cartridge cases. Modern sporting and military bullets are usually a soft lead core inside of a mostly copper (‘gilding metal’) jacket and pressed into a brass or steel case. However, bullets are not always lead. During WWI, the French Lebel 8mm ‘Balle D’ cartridge used a bullet turned from solid red brass (90% copper). In the latter half of the 20th century, much military ammunition was loaded with bullets that featured steel cores and/or steel jackets. This would be low carbon, extremely soft steel. In other words, almost all iron with very little carbon.
‘Dead soft’ steel is not an ideal metal to make bullets from, but it is quite economical, as are cartridge cases drawn from steel. Such bullets tend to be longer than lead core bullets to offset the lighter weight of steel. They are also usually expected to be used in rifles with chromium lined barrels rather than plain steel barrels. Chrome is quite hard and wear resistant.
Most firearms enthusiasts will not shoot steel jacketed or steel core bullets in fine sporting rifles if they can help it. Surplus with steel bullets is usually acceptable for military or military style rifles with chrome plated bores.