There is an interpretation of firearm in the act,
firearm—
(a) means anything from which any shot, bullet, missile, or other projectile can be discharged by force of explosive; and
(b) includes—
(i) anything that has been adapted so that it can be used to discharge a shot, bullet, missile, or other projectile by force of explosive; and
(ii) anything which is not for the time being capable of discharging any shot, bullet, missile, or other projectile but which, by its completion or the replacement of any component part or parts or the correction or repair of any defect or defects, would be a firearm within the meaning of paragraph (a) or subparagraph (i); and
(iii) anything (being a firearm within the meaning of paragraph (a) or subparagraph (i)) which is for the time being dismantled or partially dismantled; and
(iv) any specially dangerous airgun
From that a scope cant be adapted to launch a projectile, a stock can be adapted or when completed used to launch a projectile
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