M82A91 “Silver Bullet” Tank Shell
For those that aren’t familiar, these are referred to as APFSDS shells (armor-piercing fin-stabilized discarding sabot). The working bit is an extremely long, extremely thin, extremely dense fin stabilized “dart” (the bright silver thing running up the middle), which is why they are also sometimes referred to as LRPs (long rod penetrators).
This is held centered in the barrel by a multi-piece “sabot” (the darker grey bit) that separates from the dart when it leaves the gun.
The sabot is made of relatively light material, but increases the surface area that the chamber pressure can work on, which means that the dart exits the barrel much much faster than it would with a smaller diameter barrel.
These are pure KE (kinetic energy) rounds; the dart doesn’t have any explosives in it, and does all of its damage in punching through armor through sheer impact force.
The faster they travel, the better they work, which also has other advantages (flatter trajectory, faster time to target, etc). Very long/thin/dense increases the localized impact forces and decreases air resistance/velocity loss.
They’re so long that they can’t be effectively spin stabilized, which is why they have fins and are (mostly) shot out of smooth bore guns.