"Any destructive device" can cover a lot of things.
A link was provided for you to READ and RESEARCH, not post half-assed comments. From the definition immediately following firearm:
"
(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 Attorney General 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 ordinance 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 Attorney General 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, recreational or cultural purposes."
You will note the airgun is not an explosive, incendiary or poison gas.
Nor is it a bomb, grenade, rocket having a propellant charge of more than four ounces, missile having an explosive nor incendiary charge of more than one-quarter ounce, mine, nor any device similar to the previously described devices.
It is also not a type of weapon that may be readily converted to expel a projectile via an explosive or other propellant. This is key: IT USES COMPRESSED AIR OR OTHER INERT GAS.
READ. THINK. THEN REPLY. IN THAT ORDER.