History of 21% Delirium

Overview

21% Delirium is a Legendary Arc Machine Gun with the 900 RPM Rapid-Fire Frame, in the Power slot using Heavy ammo. Dexerto calls it an add-clear ace and the second-best Arc machine gun after Song of Ir Yut. Its notable perks are Overflow, which loads the weapon beyond normal capacity on Special or Heavy ammo pickup (about 230 rounds with Backup Mag, per Dexerto), and Killing Tally, a stacking damage boost. Dexerto's recommended PvE roll is Fluted Barrel, Extended Mag, Reconstruction and Killing Tally; its curated roll is Polygonal Rifling, Extended Mag, Overflow and Killing Tally.

Acquisition

Originally earned in Gambit via the "By Your Bootstraps" quest (Destiny Game Wiki). Both sources say it is no longer available that way, but they disagree on the current method: Dexerto says it is bought from the Legacy Gear section of the Exotic Archive in the Tower for 75,000 Glimmer (written when it was unsunset in The Final Shape), while Destiny Game Wiki says it is available from the Monument to Lost Lights. The pool may have changed since either article. Bungie's data marks it as not reacquirable from Collections.