UPDATE - November 18, 2015
The CSSA and CSAAA have both issued the following statement:"Despite some recent social media surrounding Beowulf magazines, the CSAAA is confirming that no ban on the sale of these magazine in Canada has been issued by the R.C.M.P. or the Minister of Public Safety at this time. All six 50 Beowulf firearms FRT numbers remain listed as legal with a five round capacity and retailers should continue to sell this product at this time:
"The 50 Beowulf Magazine
In the case of AR platform rifles chambered for 50 Beowulf calibre, the magazine is adapted from the original 5.56x45 NATO version of the magazine, generally by one or more of the following: widening the space between the magazine lips, changing the angle of the magazine lips and changing the feed angle of the magazine follower. The adaptations more efficiently feed the much larger diameter 50 Beowulf calibre cartridge. However, the original ability to contain and feed 5.56x45 mm NATO cartridges has not been deleted and the magazines remain serviceable for that purpose.
The 50 Beowulf cartridge is centrefire and the AR platform rifles which use that calibre are semiautomatic. Thus, cartridge magazines for 50 Beowulf calibre firearms are prohibited if more than five 50 Beowulf cartridges can be contained in the magazine (subparagraph ii, as above).
Magazines for the AR platform which contain four or five 50 Beowulf calibre cartridges present a more complicated situation. Such magazines will generally contain 11 and 14 cartridges respectively of 5.56x45 mm NATO (or 223 Remington) calibre. Since the 50 Beowulf calibre magazines are adapted from the original 5.56x45 mm NATO design and the ability of the magazine to perform as originally designed has not been compromised by the adaptation, such magazines are prohibited if they contain more than five 5.56x45 mm NATO cartridges. The magazines are in effect dual calibre magazines and will be prohibited if they exceed five shots capacity of either calibre.
Magazines have recently been manufactured in, or imported into Canada bearing markings suggesting they are exclusively designed for 50 Beowulf ammunition, and at four or five shot capacity, are non-prohibited magazines. This is simply not the case. All magazines for 50 Beowulf calibre AR platform firearms presently on the Canadian market are prohibited devices.
AR Platform Upper Receivers
You had also asked about 50 Beowulf calibre AR upper receivers. Your understanding is correct that they are not prohibited."
This prohibition directly contradicts RCMP Bulletin 72 determining magazine capacity, as well as a 2013 report from the Justice Department issued to Global News that specifically states Beowulf magazines to be legal devices regardless of their use:
Further details are not available, and we are reaching out to the Minister of Public Safety’s office for further comment. Possession of a prohibited device is illegal under the Criminal Code of Canada and punishable by a sentence of up to ten years.