As Broch says, when it comes to firearms, its not just the age, but also the calibre of the firearm. A gun will never be antique if the ammunition is still in production. A good example is the Lee Enfield rifle. Mass produced in the pre ww2 years, and the main ww1 rifle. Age dictates it to be antique, but the .303 cartridge is still in production in small quantities, meaning an FAC is required to own even a 100+ year old example.