Luminite Bullet

Nothing stops one of these except a wall or the clock. It runs the full length of a line of enemies, shaving a little damage off with every body it passes, and it keeps flying even after that damage has been ground down to zero. Long columns of weak enemies are exactly what it was built for, which means pillars, invasions and the packed waves of a moon event.

The way it handles invincibility frames is backwards from most piercing ammo. A single round can only score once on a given enemy, but it never locks that enemy out of being hit by anything else. The reverse is not true. If another weapon in your rotation grants long invincibility on hit, a share of your rounds will simply pass through doing nothing. Two players both firing these have no such conflict, so it is excellent in co-op and awkward next to a piercing melee weapon.

It has the biggest raw hit of any bullet, but the pierce mechanics mean you are paying for crowd damage rather than single-target damage. The right home for it is a gun that throws out a wall of rounds instead of one heavy shot, which is why the endgame machine guns are its usual pairing.