18 U.S. Code § 924
(c)(1)(A) Except to the extent that a greater minimum sentence is otherwise provided by this subsection or by any other provision of law, any person who, during and in relation to any … drug trafficking crime … for which the person may be prosecuted in a court of the United States, uses or carries a firearm, or who, in furtherance of any such crime, possesses a firearm, shall, in addition to the punishment provided for such … drug trafficking crime—
(i) be sentenced to a term of imprisonment of not less than 5 years;
(ii) if the firearm is brandished, be sentenced to a term of imprisonment of not less than 7 years; and
(iii) if the firearm is discharged, be sentenced to a term of imprisonment of not less than 10 years.
…(C) In the case of a second or subsequent conviction under this subsection, the person shall—
(i) be sentenced to a term of imprisonment of not less than 25 years; and
(ii) if the firearm involved is a machinegun or a destructive device, or is equipped with a firearm silencer or firearm muffler, be sentenced to imprisonment for life.
(D) Notwithstanding any other provision of law—
(i) a court shall not place on probation any person convicted of a violation of this subsection; and
(ii) no term of imprisonment imposed on a person under this subsection shall run concurrently with any other term of imprisonment imposed on the person, including any term of imprisonment imposed for the crime of violence or drug trafficking crime during which the firearm was used, carried, or possessed.