This glitch has now been patched as of August 20, 2020.
There have been many glitches and exploits in Warzone, but this new one is by far the most broken and lethal that we've come across.
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As part of the Season 5 update, there were some weapon balances, including a buff for the Famas. However, little did the developers know, buffing the Famas broke the weapon altogether and made it way too OP.
All you need is one attachment. A single attachment makes this weapon the most OP weapon in Warzone.
That attachment is the 12 Gauge Deputy Shotgun Underbarrel attachment. You can put whatever attachment you want in other slots, but the cause of one-shot is this specific Underbarrel Shotgun attachment.
Once you attach it, you will be able to take out an enemy player with one shot, even if they have full armor. We've been able to take out players at quite a distance, which makes it even more ridiculous.
Now, thankfully, Infinity Ward has acknowledged this issue and has released a statement on Twitter that a fix is in progress.
We’ve identified the issue with the FR 5.56 and are currently working on a fix. We’ll share more details as they become available. Thank you all for your patience.
— Infinity Ward (@InfinityWard) August 17, 2020
You can see our video below that shows how easy it is to wipe out squads using the 12 Gauge Deputy Shotgun Underbarrel attachment. You only get six bullets when you first get your loadout, but you can get a max of eight using the Munitions Box.
You can also keep refilling the ammo using the Munitions Box. Note that the refilled ammo does not fill beyond eight. Even if you have, for instance, eight bullets, then refilling won't give you more. And there is no other way of refilling the Shotgun attachment ammo.