Halo Infinite deviates from previous games in the franchise by introducing an open world to the campaign. The game starts first with levels reminiscent of classic Halo campaigns before freeing the player into this world. The first place in the open world the player finds themselves is Outpost Tremonius. Besides enemies, it houses a few secrets including a hidden gun.
To get the hidden gun in Outpost Tremonius in Halo Infinite, you will need to use your grapple and some parkour skills. Head towards the large cannons at the far side of the outpost on the right. Climb up here and go to the further cannon. Climb the barrel on the right side and you will get a prompt to pick up a weapon.
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This is the hidden gun in Outpost Tremonius. It fires like a tank and can kill groups of enemies in a single shot. Do not switch weapons or use your grapple while carrying this gun or else you will drop it and not be able to pick it back up.
You can stay on the roof and rain heavy fire from above or dive down and clear the outpost out up close. Using the hidden gun is a blast and while quickly empties the base of enemies.
For more information on Halo, check out All Halo Infinite Skulls Locations and How to claim the Halo Infinite Game Pass Bonus Rec Packs on Pro Game Guides.