As a former CFC appointed firearms verifier, I can tell you that all barrel combinations for a particular action must be reported to CFC with their unique FRT number as part of the verification. I have the Firearm Verification sheet in front of me and in section "B", Calibre/Shot/Barrel Lenght Combinations has space for 3 combinations and if more are present, to attach list. The caliber, shots, barrel length and the FRT number are required for each barrel.
The FRT number would be something like 41607-XX where the XX would be numbers corresponding to the barrel (length and gauge). For example, FRT number 41607-02 could represent a combination of 12 GA shotgun with a barrel length of 584 mm and FRT number 41607-07 could represet a combination of 22-250 Rem with a barrel length of 657 mm with both using the same frame/action represented by 41607.
I don't know what information would be included on the registration certificate (single or multiple and what information) in such situations - never saw one before but I would think that someone here must know.
I just called CFC and the lady who answered was basically concerned as to whether the classification (non-restricted, etc) of the firearm would remain the same or not with a change of barrel. She said she could not help me with such questions as to what to do if you buy another barrel and she said one of the firearms technicians would be needed to answer this.
After the transfer of the call to the technician, I was told that due to excessive call volume, that they could not answer my call and to leave my name and telephone number and that someone would call me. I did not bother to do that.
Duke1