Quite typically they are stolen from the American military by various gang members, both black and hispanic. There was a newspaper article 15 years or so about intercepted weapons that included artillery. The weapons were being taken to Mexico.
Let’s also not forget that Mexican police and Army are supplied American arms which they promptly sell to the bad guys, thus no serial numbers will ever be forthcoming to substantiate JosA’s and JosB’s (POtuS) claims.