Do you happen to have a reference for this in the firearms act? Genuinely curious. Even what the cps say is a bit confusing.
https://www.cps.gov.uk/legal-guidance/firearms
An air weapon is defined, under section 1(3)(b) and 57(4) of the Firearms Act 1968 as:
"an air rifle, air gun or air pistol which does not fall within section 5 (1) (a) and which is not of a type declared by rules made by the Secretary of State under section 53 of the Firearms Act to be specially dangerous".