And though the weapons were intended for the Afghan army and police forces, the military’s failure to document how and to where they were distributed has meant many of them found their way into the hands of militant groups throughout Afghanistan and the Middle East. In 2013, the inspector general for Afghan reconstruction reported that 43 percent of the Department of Defense’s records for small arms sent to Afghanistan were missing information or were duplicates.

This near-continuous arms flow has cost millions in taxpayers dollars, some of which remains unaccounted for, along with many weapons themselves. The Defense Department’s failure to track and maintain these weapons is a result of the convoluted bureaucracy that fuels defense contracts and Washington’s desire to build an army and police force quickly from scratch.

In recent years, the Pentagon has tried to improve its methods for tracking the weapons, but low literacy rates have made it difficult for Afghan soldiers and armorers to maintain accurate records.

Once a status symbol for senior commanders and insurgents, the American-made M16 — easily sold, lost or taken from the battlefield — is now seemingly as prevalent as the Soviet-style rifles that make up much of the insurgency’s arsenal.