The RCMP is not a lawmaker. Its opinions do not have any legal force. The
RCMP bulletin is not the law, it's just the most authoritative interpretation of the law we have, presumably written by RCMP or Justice Department lawyers who can probably be trusted to have looked at the issue very carefully. But at the end of the day it is the actual law that matters. The wording of the actual law is:
The key phrase, obviously, is "for which the magazine was originally designed." The only relevant question is whether these magazines were originally designed for cartridges of the .50 or .458 or similar cartridges or whether they were actually designed for .223 or 5.56mm.
How can this question be answered? If this issue was before a judge, the judge would require evidence related to the design.
Crown counsel would have to prove, beyond a reasonable doubt (I think), that the magazines were designed to hold the smaller cartridges, in order to obtain a conviction. The only evidence we have seen on this point is that the seller of the magazines claims they were designed for .50 calibre cartridges. If the seller is not lying, presumably the designer must have represented that the magazines were designed for .50 calibre cartridges. If the designer is going to stick to that story, then how can the Crown ever prove that the magazines were designed for smaller cartridges? Of course, maybe there are documents floating around where the designer emails someone and says, hey we need to design these for .223 and we're just going to claim that they are for .50, but it seems much more likely that the designer will be able to show that they recognized a massive demand for magazines to hold 5 .50 calibre cartridges and simply acted on that.
This is all supposition of course. The bottom line is that all that is necessary for these magazines to be non-prohibited is for the designer to plausibly claim that they designed the magazine to hold 5 .50 cartridges and a conviction for possession of a prohibited device will be impossible.
The RCMP might "decide" that these magazines are prohibited devices, but it will ultimately be up to a court. If the RCMP really does not like these, it can always try to get the government to change the regulation. A vote in parliament is not necessary for this; cabinet can change the regulation very easily.
That's my $0.02.