18 USC 922(g) says that it is illegal for various persons
to ship or transport in interstate or foreign commerce, or possess in
or affecting commerce, any firearm or ammunition; or to receive any
firearm or ammunition which has been shipped or transported in
interstate or foreign commerce.
(5) refers to "being an alien", specifically one who
has been admitted to the United States under a nonimmigrant visa (as
that term is defined in section 101(a)(26) of the Immigration and
Nationality Act (8 U.S.C. 1101(a)(26)))
with an exception
as provided in subsection (y)(2),
The (y)(2) exceptions covers foreign officials and law enforcement (not a typical tourist), and one who is
(A) admitted to the United States for lawful hunting or sporting
purposes or is in possession of a hunting license or permit lawfully
issued in the United States;
which pretty much covers simple tourists, if not typical tourists.