As can be seen this covers improvised sound suppressors, and component parts of a sound suppressor. There is no thresh hold level of sound reduction for something to fall under this definition. ATF used to require the device “appreciably” lower the sound (see Revenue Ruling 57-38); now the working definition seems to be anything that traps gas from the muzzle of the gun, or from porting of the barrel, is a sound suppressor. In general recoil compensators and flash hiders do not fall under this definition, but some designs could fall into the category. As with any borderline device the thing to do is to get a written opinion from the Technology Branch of ATF. It is what they exist to do.

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26 U.S.C. sec. 5845(f) “The term destructive device means:
1) any explosive, incendiary or poison gas
A) bomb
B) grenade
C) rocket having propellant charge of more than four ounces
D) missile having an explosive or incendiary charge of more than one-quarter ounce
E) mine, or
F) similar device
2) any type of weapon by whatever name known which will, or may be readily converted to, expel a projectile by the action of a explosive or other propellant, the barrel or barrels of which have a bore of more than one-half inch in diameter, except a shotgun or shotgun shell which the Secretary or his delegate finds is generally recognized as particularly suitable for sporting purposes; and 3) any combination of parts either designed or intended for use in converting any device into a destructive device as defined in subparagraphs (1) and (2) 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 of the United States Code; or any other device which the Secretary of the Treasury or his delegate finds is not likely to be used as a weapon, or is an antique or is a rifle which the owner intends to use solely for sporting purposes.”