https://www.polygon.com/2018/5/9/17337498/bungie-destiny-2-disable-rat-king-prometheus-lens-exotic-bug

Rat King, one of the newly updated exotics in Destiny 2: Warmind, has recently been disabled on live servers. While we’re not entirely sure how the exploit is used, players have been able to glitch their magazines to hold more ammo than they’re supposed to. Rat King is apparently the culprit and has been disabled, meaning that players cannot equip the sidearm.

Bungie announced it was disabling the exotic sidearm on Twitter today.