The Sons of the Forest rifle is a new addition to the brutal survival game and was added with the latest update. Up until now, you've been able to choose from a range of weapons, but this is the first time you'll get to add a rifle to your arsenal. At the very least, it should be handy for dealing with any hungry locals that stray too close.
Also included with the update are drivable golf carts, which you can drive pretty much anywhere on the island that the terrain allows. Having to set aside enough supplies for a day or more trek across the island might be a thing of the past, and who knows, maybe you've already grabbed one and can use it to get to the Sons of the Forest rifle location without breaking a sweat. Here's where to find it.
Sons of the Forest rifle location
You'll need to venture into a cave to find the rifle and you'll need the rebreather to reach it. The cave in question is to the east of the snowy area in the middle of the island, on the edge of the large lake. You can find the exact location marked on the map above.
Once inside the cave, keep going until you find a small area of deep water you can swim down into using the rebreather. The cave itself is quite large, but if you keep heading toward the back of the cave and keep heading towards the lowest point, you'll eventually find the correct spot. Follow the long underwater tunnel—try to ignore the dismembered body parts as you swim past them—and pull yourself out the other side.
You'll find the body of a guy slumped against the cave wall here, so loot what you can from him then keep that same wall on your right and head forward. Keep going, sticking to the right and you'll eventually find another guy just past an area that's fairly lit up. This one is holding the rifle so loot the weapon and the rifle ammo on the floor next to him. Good job, now get shooting.