The Rope Gun is something akin to a grappling hook. It can be found at the end of the cave, not far from the Stun Baton. It can be used on specific ropes in a few caves to cross gaps that wouldn't otherwise be possible without it, as well as to create your ziplines. As of release 1.0, the rope gun has a zipline range of 150 meters. Walking beyond 150 meters will break the rope and trying to aim beyond 150 meters will give a red indicator on rangefinder.
Zipline Rope - Sons of the Forest Wiki
Sons of the Forest rope gun location—find the zipline
How to Get the Ropegun Grappling Hook- Sons of the Forest
Item Id - Sons of the Forest Wiki
Crafting - Official The Forest Wiki
How to Get the Zipline Rope Gun - Sons of the Forest Guide - IGN
Silencer - Sons of the Forest Wiki
Large Effigy, The Forest Wiki
Stun baton - Sons of the Forest Wiki
Sons of the Forest Wiki
How to Get the Zipline Rope Gun - Sons of the Forest Guide - IGN
Sons Of The Forest - How To Get The Rope Gun - GameSpot