KYIV, Ukraine—When Maxim Masur deployed to the front lines of the Ukraine conflict last June with the Aydar Battalion, his unit only had one assault rifle for every 10 men.

"We had to take Kalashnikovs off of the dead enemy," said Masur, 25, who served near the eastern Ukrainian town of Luhansk. "We had no other choice if we wanted to live."

Despite being outgunned by pro-Russian separatists, who he claimed were being supplied by Russia, Masur survived and returned to Kyiv to train new recruits.

But it is unclear where the assault rifles that Masur and his comrades confiscated on the battlefield ended up, highlighting the shadowy origin and fate of many of the weapons used in the year-old Ukraine conflict.