An extensive web of laws applies to firearms transactions related to gun trafficking between the United States and Mexico.
First, any Mexican or other foreign citizen is generally prohibited or restricted from acquiring firearms in the United States. Federal law prohibits illegal aliens (along with other prohibited persons, such as convicted felons) from possessing or receiving firearms. Lawful resident aliens may acquire firearms in the U.S., but under ATF regulations must prove 90 days’ continuous residence in a state to do so. Nonimmigrant aliens must meet the same 90-day residency requirement, and must also fall under one of the other exceptions to the federal statute that generally prohibits nonimmigrant aliens from possessing or receiving firearms. Transfers to aliens who don’t meet any of these requirements are prohibited. Sales by licensees are subject to instant background checks, which check the buyer’s alien status along with criminal and other records.
Prohibited aliens may try to evade these restrictions by using “straw purchasers” who are not prohibited from receiving firearms. A straw purchaser who buys from a licensed dealer violates the federal law that prohibits making false statements in dealers’ records in connection with the acquisition of a firearm, because the “Firearms Transaction Record” completed by the buyer asks if the person completing the form is the “actual buyer of the firearm(s) listed on the form.” Even a person who simply travels into a state from any other “State or foreign country” with intent to engage in such a transaction violates federal law.
On the dealer’s side of the table, any person who sells or delivers a firearm to a person the transferor knows or has reasonable cause to believe is not a resident of the same state would generally violate the law. A licensee who makes sales off the books also violates the law by failing to keep required records of transactions.
Transporting firearms from the U.S. to Mexico poses more legal obstacles to the would-be trafficker. Shipping, transporting or receiving a firearm with intent to commit a felony with it, or knowing that someone else will use it to commit a felony, is punishable by ten years in prison. The trafficker would also break the law if he tried to ship firearms by common carrier without declaring the contents of the package, or by mailing a handgun. There are also specific prohibitions on shipping or transporting stolen firearms, or firearms with obliterated serial numbers.
If a trafficker manages to export firearms from the U.S. without a license from the State Department, he violates the Arms Export Control Act, which prohibits unlicensed exports of “defense articles.” Illegally exported “arms or munitions of war” are subject to seizure and forfeiture.
Even if the trafficker does not remove the firearm from the U.S., he can still be punished for transferring it to someone else in the U.S., if the transferor knows the firearm will be used to commit a violent or drug trafficking crime. And even if a group of traffickers are ultimately unsuccessful, they can still be punished under the general federal conspiracy statute, as long as one of the conspirators commits some act to carry out the plan.
Mexico, of course, also has strict and complex gun laws of its own, which allow for up to three years’ imprisonment for importation of arms “that can only be used to attack” or that are not “for public servants.” Mexico prohibits civilian possession of firearms or ammunition in calibers commonly used by the military (such as 9mm or larger handguns, and .223 and .30 caliber rifles). Unauthorized possession of these and other “arm[s] exclusively reserved for the use of the Army, Navy or Air Force” is punishable by up to fifteen years’ imprisonment. Gun sales within Mexico are a government monopoly, conducted at a single store operated by the army in Mexico City.
Another factor in all cross-border issues, of course, is corruption among Mexican officials. Some in the Mexican government, including the legislature’s National Defense Commission, have acknowledged this. According to one Mexican congressman, “[t]he corruption in customs, and the incapacity of the Mexican state to control it, allows them right through.”
This summary should make clear that the United States has very strong criminal laws that apply to every link in the trafficking chain, between the first acquisition of a firearm in the U.S. and its transportation to Mexico or any other country. Strong enforcement of existing U.S. firearm laws, and cooperative enforcement programs with Mexican authorities, are likely to be more productive than added restrictions on firearm transactions by U.S. citizens.
 18 U.S.C. § 922(g)(5)(B), (y)(2) (providing exceptions for persons admitted for lawful hunting or sporting purposes and persons in possession of a U.S. hunting license, as well as foreign diplomats, law enforcement and other officials).
 18 U.S.C. § 922(t); Federal Bureau of Investigation, Criminal Justice Information Services Division, National Instant Criminal Background Check System (NICS) Operations 2005 iv (Jan. 2006). The main exception to the background check requirement—an exemption for buyers who present a state license to possess or acquire a firearm—wouldn’t be available to prohibited aliens or other prohibited buyers, because the exemption only applies to permits issued after a background check.
 See 18 U.S.C. § 922(a)(5) (restricting sales by nonlicensees) and (b)(3) (licensees). While the statute does allow some interstate sales of rifles and shotguns by licensees (only), those sales must comply with the laws of both the seller’s and the buyer’s “States,” so sales to a resident of Mexico would also be prohibited. There is no interstate sale exception for handguns.
 Ley Federal de Armas de Fuego y Explosivos of Jan. 11, 1972 (D.O., Jan. 11, 1972), arts. 10 & 11 (English translation as amended through June 2002, available at http://www.natlaw.com/trans/tnstcs1.htm; for another translation based on Mexico’s 2004 text, see http://davekopel.org/Espanol/mexican-firearms-statutes.htm).