Supply or sourcing of weapons to terorists

The peace loving people of the world want to know how it has become possible for the terrorists to acquire the most modern assault weapons. Most of these weapons technology originated in the advanced western nations. So much talk is going on about nuclear proliferation etc. But what are the ground actions being taken to limit the sourcing of these weapons by the terrorists. There is a need for specific deterrent action by all UN countries particularly by UNS