I thought it'd be a good idea for people to post their weapon farming routes to help others to get some decent loot in short time. Also maybe point out what kind of weapons you have found on your route. The one I have been using recently is:
Start off at the Warehouse in Moresby as your checkpoint (accept a quest here to make the game save there as a check point). First chest is in the warehouse.
Next, fast travel to the River Village in the Jungle and open the chests in the room behind the fast travel they are both level 1 locked, one weapon chest and one gun chest. Use the trader here to sell any weapons you don't want and make some cash (especially from firearms).
Next fast travel to Overpass Camp and there are two chests next to the trader.
Fast travel to Jungle, Antenna and there is a level 2 locked chest on the roof of the compound. There's also a level 3 locked gun chest in the corner of the compound. It's directly right when you go through the gate, you have to climb up a small ladder onto a container and drop down to it. There is another chest in the building but I never find anything better than white weapons so I skip it, open it if you want.
Save the game by changing sensitivity in Options and then load last checkpoint and start over.
I have found oranges in the River Village, one of which was the unique Blunt Speaker. I have found a couple of Oranges in Overpass Camp and the level 3 gun chest in Jungle, Antenna nearly always contains a coloured weapon, many of which are valuable.