Quote Originally Posted by endure View Post
spring - a piece of curved or bent metal that can be pressed into a smaller space but then returns to its usual shape:

A spring is only a FAC spring when it's fitted to a gun. Otherwise it's just a piece of twisted metal.
And even that would depend on the situation,

Perhaps this spring would fit a different rifle where it would be under 12fpe

Only once a collection of parts are assembled in to a working item does that collection of parts become "an air rifle" at which point the owner/builder must ensure that it complies with UK law.