Why arent bullets made of iron?
Sometimes they are!
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.
Use guns but I am allergic to lead
The impact of a bullet is 1/2 m V^2
so you want to maximize m which is the mass.
You also want to keep the bullet small so that the effect of wind is minimal and the gun barrel diameter (the caliber) is small.
So you use the densest material that is readily and cheaply available. Also being soft and conforming to the rifling helps the guns last longer, and makes the bullet mushroom out on impact to do the most damage to the target.
Lead fits those criteria, iron much less so.
Have been a shooter for 52 years and own a wide range of firearms.
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.
Military veteran, hobby shooter
As others have indicated, iron is both too light and too hard. This is also why you don’t see titanium or aluminum bullets.
Interestingly, you do sometimes see copper ones - they don’t contain lead, so they don’t have the various environmental problems that lead can pose, and since copper is also fairly soft, they deform well enough. Copper is both harder and lighter than lead, and it’s more expensive, but if you can’t use lead, it’s probably the second-best metal for handgun and rifle bullets. It also holds up better when shooting through wood paneling, car doors, glass, etc, which is important in a military or police context. (Soldiers and cops might have to shoot through concealment to get at a bad guy hiding behind it, and copper bullets are pretty good at that. Cover still beats them, which is why it’s cover, vice concealment, of course.)
What would the best be? Well, if cost and supply are no object, there’s nothing quite like gold. Why? It’s almost as soft as lead, and it’s a lot denser. Gold bullets would work very well indeed. In cases where you don’t need a bullet to be soft, but density is ultra-important, you’ll sometimes see tungsten or uranium used, since both of these are hard and very dense. In a cost-is-no-object world, one might use iridium instead.
For shotguns, where you need something cheap and heavy, but it doesn’t necessarily need to be soft, bismuth is the metal of choice. It’s not that cheap - much more expensive than iron or copper - but it’s almost as dense as lead. This was a problem when regulations started restricting the use of lead birdshot, due to poisoning of waterfowl. The earliest solution to that was, in fact to use iron or steel shot, but this wasn’t very good: the light shot slowed down quickly and was both short-range and didn’t inflict quick, clean kills, and the hardness of the shot tended to tear up barrels. Eventually, bismuth shot was introduced and it works pretty well for most purposes, though everyone complains about the cost.
Note that’s only for birdshot - buckshot and slugs are still almost always lead or copper-plated lead. Solid copper slugs exist, but are pretty rare.
MD from University of Mississippi Medical Center (1978)
Lead alloys are generally preferred, and copper is becoming more common. These have properties more useful.
They’re dense, so there’s plenty of weight in a compact size. The added momentum is useful once it gets to the target, when used for military, defensive or hunting purposes.
When they go down the barrel, they start out a little larger than the barrel. For instance, the common 30 caliber rifle has a bore of 0.30″ with rifling (grooves) cut into it that are about 0.04″ deep. The bullet is 0.308–0.309″ diameter, and it engages the grooves, forming itself to that shape. Iron wouldn’t do this well, and it would be hard on the barrel. There’s enough trouble “shooting out” barrels after a few thousand rounds even with the softer metals.
Economist, Forecaster, Applications Specialist, CSCP, Gear Head
As far as firearm projectiles go, many states have outlawed lead shot for fowl hunting. My home state of Virginia only permits steel shot for bird hunting. Virginia's wetlands were being poluted by all the lead shot blown into the environment by hunters.
Some hunters bemoan this law. Lead is denser and flies further with more energy than steel. However it's also poisonous.
Bullets can be made of steel. In fact, “block buster's” are armor piercing rounds made of steel with a copper nickel jacket to prevent bore wear.
They do use it but only in military ammo. They make both steel cored and steel jacketed copper washed bullets. It saves money because lead is expensive and harder bullets penetrate better.
Registered Professional Engineer, BSAeroE, MSME, MBA, Pilot
The primary characteristics of bullets are that their material should be dense to retain energy in atmospheric trajectory, strong enough to hold together under the acceleration forces due to firing, yet soft enough to not damage the rifling inside of the barrel. Lead is a good material, iron is not (not dense enough, not soft enough).