The international small arms surveys used to tabulate gun ownership by country seem to treat "registered" as meaning the government has records of firearms legally manufactured in, or imported to, the country not necssarily that these guns are registered with the government by the possessor
In a report to one of the small arms surveys, Mexican security expert Georgina Sanchez listed:
3 million registered civilian firearms in "civilian calibers", registered by owner with the Mexican Army.
12 million unregistered military caliber weapons in civilian hands.
40 million unregistered civilian caliber drearms in civilian hands.