There are a bunch of weapons in Halo Infinite and many of the weapons have a more powerful variant you can unlock. You unlock these variants by defeating high-profile enemies. After defeating one of these enemies, go to a FOB to equip the weapon. The Purging Shock Rifle is an upgraded version of the Shock Rifle that has higher damage and a larger area of effect when you hit an enemy. Here is how you unlock the Purging Shock Rifle in Halo Infinite.
The Purging Shock Rifle is obtained by killing the Skimmer Alpha. These enemies are new to Halo Infinite and love to fly around. The Skimmer Alpha can be found after completing the Pelican Down mission and starting The Sequence mission. Head to the area on the map above. You can easily get there from FOB Juliet if you have it unlocked.
The Skimmer Alpha can be found tucked away in a little cave. It is easy to defeat, and a well-placed sniper shot can take it out with one hit. The other enemies in the area are more skimmers and a few brutes. Once they are all defeated, you will unlock the rifle. Near where you killed the Skimmer Alpha is a container with the Purging Shock Rifle inside.