Some collected locations of chests in The Dahl Headlands.
Map of the Weapons Crates
Location 1: Lucky's Last Chance Watering Hole
Related Mission: Getting Lucky
At the Outpost, you can see two tires stacked on one another on the opposite side, left of the Bounty Board (if you are facing towards the Bounty Board). Jump on it. Then turn around. You should see the Bounty Board has a guard over it. Jump on that. Then jump on the roof. There should be a white weapon crate down the end of the roof.
Locations 2 & 3: Lucky's Last Chance Watering Hole
Related Mission: Getting Lucky
As you come up to the entrance where you must go through to get Lucky, fight your way through the first area. As you come up to the entrance to the second area, you should see a fence. DON'T go through yet. Somewhere along that fence is a white weapon crate.
When you've finished there, turn around and go left. There should be a wall there. Again, somewhere along that wall is a red weapon crate.
Location 4: Ludicrous Speedway
Related Mission: Death Race Pandora
At the start of the Ludicrous Speedway, there is a small outpost with a vending machine and a Catch-A-Ride terminal. Now, go to the vending machine and jump on it. If you can't there is a pipe that can help you. Once you are on the vending machine, jump on the roof of the outpost. Go to the end of the roof and there is red weapon crate there.
Location 5: Top-left of the Dahl Headland map
Related Mission: Bandit Treasure: X Marks the Spot
At the top of the map is the bandit's treasure. Look to the left a bit more and you can see a bandit outpost. On the map it looks like the space in a spanner. There are guards there. Once you have killed the guards, there are TWO red weapon chests at the end behind some tin cover.