> Please source the claim that the legal purchase of firearms by US citizens from FFLs, such as Walmart, is a significant contributor to the presence of firearms in Central America.

For example, one study is at https://nacla.org/article/small-arms-trade-latin-america.

> The U.S.-Mexican border is also a central route through which illicit small arms enter Latin America. A study released by the Mexican government suggests that as many as 2,000 guns are crossing the U.S.-Mexico border daily. As in Colombia, these guns are fueling an arms race, in this case between Mexican drug cartels, costing the lives of 4,000 people in 18 months.14 Weapons, including assault rifles like AK-47s, AR-15s, and M-16s, fetch up to three times their U.S. market value in Mexico, assuring a continued southward flow of weapons.