This article[1] sheds some light on small arms proliferation. Bulgaria and Serbia aren't even mentioned. While there are still lots of unlicensed rifles in the West Balkans[2], the bulk of small arms still comes from the US, Russia, China and Western European states. They're sold to dictators or given to rebel groups in conflict areas. The former are then stolen, smuggled and resold to rebel groups and civillians.

  1. https://social.shorthand.com/ymuntaiwan/ug53Pq2PhT/disec-i-proliferation-of-small-arms-and-light-weaponry
  2. http://www.smallarmssurvey.org/fileadmin/docs/G-Issue-briefs/SAS-AV-IB4-Western-Balkans.pdf