THe FBI estimates at least 200 million firearms are owned privately by U.S. citizens. The number fluctuates because others speculate at least 100 million guns are owned illegally, and most guns are owned by people who have more than one.
Here's a link to a gun forum where they discuss the very same question.
http://www.northeastshooters.com/vbulletin/threads/184347-How-many-privately-owned-firearms-in-US
If you google, "300 million owned guns U.S.",you'll get over 100 million hits.
Here's a wiki quote from Wiki on the Mexican Drug War;
Mexicans have a constitutional right to own firearms,
[100] but legal purchase from the single Mexican gun shop in Mexico City is extremely difficult.
[101] A significant number of firearms that make their way to Mexico come from U.S. gunshops. These are then smuggled into Mexico across the US-Mexico border.
[102][103] Most grenades and rocket-launchers are smuggled through
Guatemalan borders
[104] or stolen from the Mexican police or military.
[105]
The vast majority of the handguns and many of the assault rifles used by the cartels enter Mexico from the United States.[104] Consequently, black market firearms are widely available. The most common smuggled firearms include
AR-15 and
AK-47 type rifles, and
FN 5.7 caliber semi-automatic pistols. In 2009, Mexico seized more than 4,400 firearms of the AK-47 and AR-15 types.
[106] Grenade launchers are known to have been used against Mexican security forces, and
M4 Carbines with
M203 grenade launchers have been confiscated.
[107] It is believed that some of these high powered weapons and related accessories may have been stolen from U.S. military bases.
The ATF estimates that over 70% of the guns found in Mexico come from the U.S.;
http://thehill.com/blogs/blog-brief...-percent-of-guns-found-in-mexico-come-from-us
Remember Fast and Furious??? Opponents of Obama's administration have misrepresented what this operation was about. The ATF wasn't selling guns to Mexi gun runners. They were surveilling straw buyers along the U.S./Mexican border and attempting to trace where those guns ended up.
One of those guns bought at a U.S. gun shop was used to kill a U.S. Border Patrol agent.
Google is your friend. No one's making up stats or numbers. Most stats I quote can be easily searched for corroboration.
Bottom line, there are way too many guns in the U.S. and Uncle Sam is THE major arms dealer for the drug cartels in Mexico.