There's a tank cannon you can 'pick up' at the end of Outpost Tremonious.
At the end of the level, you have to grapple up onto the UNSC ship, press the activate button by the AA cannon, then climb up on the cannon furthest from you and you should be able to swap your current weapon for a tank cannon
At the end of the level, you have to grapple up onto the UNSC ship, press the activate button by the AA cannon, then climb up on the cannon furthest from you and you should be able to swap your current weapon for a tank cannon
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