Halo Infinite’s campaign has many objectives for players to complete and secrets to uncover. These range from main missions to progress the story as well as optional side missions. One type of side mission is to track down variant weapons such as the Unbound Plasma Pistol.
You can find the Unbound Plasma Pistol in the same region the Tower is in to the northeast. To unlock it, you will need to defeat the high-value target, Briglard. Once you have defeated Briglard and claimed the pistol, you can now requisition it from any of your FOBs. The Unbound Plasma Pistol will fire a volley of plasma energy rather than a single shot.
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Note that these high-value targets and their variant weapons do not seem to spawn until you have found traces of them in your database. Capturing FOBs will reveal more high-value targets and let you claim their variant weapons.
So while you may be able to head to this location early, it seems that you will still need to acquire Briglard as a target in your database before you can unlock the Unbound Plasma Pistol.
For more information on Halo, check out How to activate skulls in Halo Infinite Campaign and Where to find the second Skull location in Halo Infinite Campaign – Cowbell Skull on Pro Game Guides.