You’re going to want to raid mule camps to get the most metals and ceramics in the game. Here’s how you can do so with as little difficulty as possible. First, stock up on smoke decoys and stun bombs. If you haven’t unlocked the latter yet, bola guns should be fine. Three of each item should be enough. Get yourself a vehicle parked in a facility garage as well. For this strategy, you’re going to need a truck, since they can carry the greatest amount of cargo in the game. Now do the following: drive to the outskirts of a mule camp (orange circle on your map), and go a little past one of the camp posts. Wait there. You should be pinged by the mules and they will head towards your location shortly thereafter. If you’re lucky, a group of 4-5 of them will be headed your way. At this point, quickly get out of your truck, drop a smoke decoy and remote stun bomb near it, and hide somewhere nearby where the mules won’t be able to see you. They’ll open the decoy and be hit by the smoke. While they’re distracted, you should take them all out by detonating the remote bomb, or by shooting them with your bola gun if that’s what you’re carrying instead. Make sure you knock out any mules who aren’t unconscious yet.

Scoot around the camp and repeat this process. Get pinged, drop a decoy, hide, let the mules come, and take them out either with the bomb or the gun. There will probably be a few remaining mules here and there who weren’t lured into the trap but at that point they should be very easy to deal with now that you’ve taken out a bulk of the camp. You can also use your truck for cover as the last few mules come trickling towards you. With all the mules out of the picture, it’s time to loot. Get back into your truck, drive around the camp, and pick up as many of the ceramics and metals as you can. Ceramics often come in a stack of 320, offering you copious amounts, while metal caps out at a little over 200 (as far as I know). That’s A LOT, considering how many of these stacks you are going to be picking up around the camp. Load these supplies into your truck, and then head for the auto pavers around the map to build your roads. Mule camps take a few hours to fully repopulate, but you can spend this time withdrawing materials from facilities, completing deliveries, and picking up lost cargo to earn these sweet materials on the side while you wait for the mules to return. Have fun, and keep on keeping on!
