USA. 1968 ‘Definitions - Firearm.’ Gun Control Act of 1968, Public Law 90-618, Title 18, United States Code - Firearms; Chapter 44 (Section 921(a)(3)). Washington, DC: Congress of the United States. 1 January

Relevant contents

Section 921. Definitions

(a) As used in this chapter…

(3) The term 'firearm' means

(A) any weapon (including a starter gun) which will or is designed to or may readily be converted to expel a projectile by the action of an explosive;
(B) the frame or receiver of any such weapon;
(C) any firearm muffler or firearm silencer; or
(D) any destructive device.

Such term does not include an antique firearm.

(4) The term 'destructive device' means —

(A) any explosive, incendiary, or poison gas —
(i) bomb,
(ii) grenade,
(iii) rocket having a propellant charge of more than four ounces,
(iv) missile having an explosive or incendiary charge of more than one-quarter ounce,
(v) mine, or
(vi) device similar to any of the devices described in the preceding clauses;

(B) any type of weapon (other than a shotgun or a shotgun shell which the Secretary finds is generally recognized as particularly suitable for sporting purposes) by whatever name known which will, or which may be readily converted to, expel a projectile by the action of an explosive or other propellant, and which has any barrel with a bore of more than one-half inch in diameter; and

(C) any combination of parts either designed or intended for use in converting any device into any destructive device described in subparagraph (A) or (B) and from which a destructive device may be readily assembled.

The term 'destructive device" shall not include any device which is neither designed nor redesigned for use as a weapon; any device, although originally designed for use as a weapon, which is redesigned for use as a signaling, pyrotechnic, line throwing, safety, or similar device; surplus ordnance sold, loaned, or given by the Secretary of the Army pursuant to the provisions of section 4684(2), 4685, or 4686 of title 10; or any other device which the Secretary of the Treasury finds is not likely to be used as a weapon, is an antique, or is a rifle which the owner intends to use solely for sporting purposes…

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http://www.gpo.gov/fdsys/pkg/USCODE-2011-title18/pdf/USCODE-2011-title18-partI-chap44.pdf