Weapons can often take a strange path as they make their way to their ultimate destination. An influx of Croatian weapons has made their way into ISIS hands due to the actions of several countries and organizations. Croatia sold weapons to Saudi Arabia who in turn armed Syrian rebels. ISIS is also fighting the Syrian state. Thus ISIS troops in Iraq now have Croatian weapons. Planes have also been known to make the same route from Croatia to Iraq by way of Turkey. For further details on the flow of arms from Croatia to the Middle East, you can read a detailed take on the situation courtesy of the New York Times here.
The RG-6 is one such weapon that has made its way into Iraq by way of Croatia. Like many of the weapons that have fallen into ISIS hands, the RG-6 is originally a Russian design. The grenade launcher first saw combat during small scale urban conflicts such as the Chechan Wars. The RG-6 holds six rounds which can be fired at two rounds a second. It has a firing range of 400 meters.