The visualization is driven by the Stockholm International Peace Research Institute (SIPRI) database and it tracks only what is calls "major weapons" (see section 3 in https://sipri.org/databases/armstransfers/sources-and-method...). So small guns or similar arms will not be covered. Also, I have not checked the dataset yet, but from the description, a chord diagram of source and destination nations could probably be plotted from the SIPRI database. Would make for a good spare-time dataviz project!