Here is another twist on breaking a M-14 for its parts.
Apparently, if a 12(3) CA is dismantled completely, the stripped receiver no longer has 12(3) CA status. It becomes 12(2) - full auto - but is not and cannot be registered as such. This situation was first reported when someone attempted to transfer a stripped M-14 receiver from a dismantled 12(3) rifle.
This is similar to the situation respecting 12(6) pistols. Rebarrel one and it can be reregistered as restricted. Remove the restricted length barrel, and the receiver reverts to 12(6), but cannot be reregistered as such.
This makes little sense given that the receiver on it's own is only a wepon if thrown at someone or used as a short club. In the case of the M14 a few additional and unaltered parts are required to make a 12-3 select fire again.