I have seen a Bren that was rebuilt from a demilitarized MK. II Long Branch. After the receiver was repaired, the gun was duly registered, no problems. A new barrel, bolt and gas piston had been installed. Looked good, and was functional. No changes were required in the mechanism. It functionned in the normal open bolt manner, and the trigger mechanism was intact. Of course, this was in the 1960s, and the gun was registered as full automatic, and would be a 12(2) now.
While it is RCMP practice to consider a receiver made using parts from a demilled receiver to be in the same class as the original receiver was,
there are a few options available:
-ask for a formal ruling if a semi auto Bren made by a particular US manufacturer is acceptable. This could require going through the process of importing one, and submitting it for testing and a decision;
-ask for a formal ruling on whether a Bren receiver made in Canada using some or all parts from a demilled gun would be accepted as a non-restricted receiver.
Manufacturing a Bren receiver from a 40 pound bar of steel would be a major undertaking for a very well equipped machine shop. The 247 separate operations Stencollector referred to were operations made using the specialized, purpose built tooling.
To the best of my knowledge, the only receiver which has been accepted by the RCMP which uses original receiver parts in its manufacture is the 1919 type Browning. This is only because they accepted the position that the right hand sideplate is the receiver. There are various rulings that have been made with respect to exactly what part of the gun is the receiver: breeched barrel for percussion muzzleloaders, lower receiver for ARs, upper receiver for FNFALs, magazine housing for Sten types, right hand sideplate for Browning guns.