This is what you must comply with to purchase a firearm using any law enforcement exception:
27 CFR 478
§ 478.134 Sale of firearms to law enforcement
officers.
(a) Law enforcement officers purchasing
firearms for official use who provide
the licensee with a certification on agency
letterhead, signed by a person in authority
within the agency (other than the officer
purchasing the firearm), stating that the
officer will use the firearm in official duties
and that a records check reveals that the
purchasing officer has no convictions for
misdemeanor crimes of domestic violence
are not required to complete Form 4473 or
Form 5300.35. The law enforcement officer
purchasing the firearm may purchase
a firearm from a licensee in another State,
regardless of where the officer resides or
where the agency is located.
(b) (1) The following individuals are
considered to have sufficient authority to
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certify that law enforcement officers purchasing
firearms will use the firearms in
the performance of official duties:
(i) In a city or county police department,
the director of public safety
or the chief or commissioner of police.
(ii) In a sheriff's office, the sheriff.
(iii) In a State police or highway
patrol department, the superintendent
or the supervisor in charge of the office
to which the State officer or employee
is assigned.
(iv) In Federal law enforcement offices,
the supervisor in charge of the
office to which the Federal officer or
employee is assigned.
(2) An individual signing on behalf of
the person in authority is acceptable,
provided there is a proper delegation of
authority.
(c) Licensees are not required to prepare
a Form 4473 or Form 5300.35 covering
sales of firearm made in accordance
with paragraph (a) of this section to law
enforcement officers for official use. However,
disposition to the officer must be
entered into the licensee's permanent
records, and the certification letter must be
retained in the licensee's files.