How does the armor-piercing bullet work?

There are several different technologies with armor-piercing bullets.

A common one is for the bullet to have a thin core of steel, or other hard metal. Typically bullets have soft lead surrounded by soft copper. When these bullets hit something, they deform, usually increasing their diameter as they penetrate. Most armor exploits this property.

Steel will not deform like lead & copper, and is much more likely to penetrate deeper than a regular bullet.