Why lead is used to build bullet and not other, harder metals?
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To maximize performance of the bullet, it has to seal VERY tightly to the inner bore of the barrel (and chamber rpessures are very high, tens of thousands of PSI or more). To do this we rely on a process called “obturation”, basically the bulled gets squeezed down a bit as it enters the barrell, and the otuer surface of the bullet deforms until it conform perfectly to the bore, sealing off all the high rpessure gas behind it.
For this to work effectively, the metal has to be soft enough to deform easily. Lead is very soft. Lead also has the benefit of being very dense (so you can heavier bullets, which has ballistic benefits), and also having a very low melting point (so that you can melt lead into a simple mold and make your own bullets over a campfire or whatever). So it was a great option in the early days of guns (and still is for many uses).
However as more modern guns reached higher and higher bullet velocities, and mdoern gunpowder burned with higher pressures, we found that lead started to have some problems. It was almost TOO soft and had TOO low a melting point, and in high eprformance rifles you might have problems shooting sold-lead bullets. So we developed metal “jackets”. This is a thin layer of some other metal (usually a copper allow, soemtimes steel, etc) that is tougher and harder than lead, but still thin/soft enough to do the sealing/obturation part. These will often still have a lead “core” inside them, because lead is still very dense, and its cheap, and being so sfot can make the bullet expnd more on impact, doing more damage. but there are also modern “monolithic” copper rounds that are just a single piece of pure solid copper. There are also some specialty rounds that would include otehr metal cores. This might be a tungsten ‘penetrator’ to help the bullet go through armor, or sometimes just a piece of mild steel (fairly soft) that was included to give the bullet the correct weight or center of gravity or other reasons.
For guns chambered in older calibers, or guns that inherently shoot at lower velocities and pressures (many pistols/revolvers, etc) you can still use plain old solid cast-lead bullets, and they are a very economical choice if so. But in most cases a copper jacketed bullet is going to be a better choice overall.
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