If you have access to the FRT table, you can look up #121656. it is the number for a 50 Beowulf rifle, But to save you some time:
The "Canadian Law Comments" section says for FRT #121656.
In the case of AR platform rifles chambered for the 50 Beowulf calibre, the magazine is adapted from the original 5.56x45 NATO version of the magazine but the ability of the magazine to perform as originally designed has not been compromised by the adaption. As a result such magazines are prohibited if they contain more than five 5.56x45 NATO cartridges. The magazines are in effect dual calibre magazines and are prohibited if they exceed five shots capacity of either calibre. An AR platform magazine limited to no more than five 5.56x45 NATO (223 Remington) cartridges will hold no more than two 50 Beowulf cartridges.
not saying I agree with this, but that is in the table now, the people who wrote it are the sole authority on all gun legalities in Canada since the Liberals came to office.