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I do not think you will find an official written definition. This was omitted when the FA was written.
In practice, I believe the breeched barrel is considered the equivalent of a receiver.
I have gone through this several times in the past couple of years with the CFC when registering caplock rifles and a pistol that I built new and I can't imagine the answer would be different for an antique because all the main parts are the same. Once they understood there wasn't any so-called "action" then we have always agreed that the barrel is the primary component in lieu of an action. Thus, I attach the sticker or engrave the Firearm Identification Number on the barrel (bottom flat). Hope that helps.
a detached barrel is a spare part and in many, if not most cases cannot be defined as a percussion or a flint barrel. In the case of modern long arm barrels that means that you cannot define it as modern or antique in a legal sense