Staples v. United States, 511 U.S. 600 (1994), was a case where the United States Supreme Court ruled that the crime of possessing an unregistered machine gun requires knowledge that the firearm is a machine gun under the National Firearms Act.

During a search warrant on Harold Staples' residence by the BATF, agents recovered an AR-15 rifle with a filed metal stop (that normally prevents an M16 selector switch from rotating to the full auto position) and several M16 parts.

The trial court rejected his request to include a defense of ignorance in the jury instructions, and he was subsequently convicted.

In a 7-2 opinion delivered by Justice Clarence Thomas, the Court ruled that due to historical tradition of private individuals owning firearms, guns are not as dangerous as hard drugs or hand grenades.

In the case of the latter, such "public welfare" offenses consider means rea as the knowledge that the defendant was selling or possessing such items.