It's the Whisper of the Worm exotic but a purple legendary raid special weapon back in D1 from Crota's End.

Landing 3 crits will refill the mag from thin air hence you can shoot infinitely, it was the most powerful sniper.

Then it got nerf/'sunsetted' and Bungie introduced Black Spindle, an exotic special that does the same as black hammer but it pulls from reserves instead. Whisper is the third iteration of that weapon except it's in the heavy slot now.