From the RCMP website.
Q7. What is the maximum number of cartridges that a firearm magazine can legally hold?
A7. As set out in Criminal Code Regulations, some large-capacity magazines are prohibited regardless of the class of firearm to which the magazines are attached. As a general rule, the maximum magazine capacity is:
5 cartridges for most magazines designed for a semi-automatic centre-fire long gun; or
10 cartridges for most handgun magazines
A large-capacity magazine is not prohibited if it has been permanently altered so that it cannot hold more than the number of cartridges allowed by law. Acceptable ways to alter a magazine are set out in the regulations.
There is no limit to the magazine capacity for semi-automatic rim-fire long guns, or for other long guns that are not semi-automatics, with some exceptions. See also Special Bulletin for Businesses No. 72, Maximum Permitted Magazine Capacity.
Here are the exceptions spoken of above.
Current Issues
1. Magazines designed or manufactured for both rimfire calibre rifles and handguns
Magazines designed to contain rimfire cartridges and designed or manufactured for use in a rifle do not have a regulated capacity. However, magazines designed to contain rimfire cartridges and designed or manufactured for use in a semiautomatic handgun are limited to 10 cartridges. Magazines designed or manufactured for use in both rifles and semiautomatic handguns are subject to the handgun limit of 10 cartridges.
Example:
Smith & Wesson M&P 15-22 rifle and 15-22P pistol chambered for 22LR caliber:
the 10 round magazine is unregulated
the 25 round magazine is a prohibited device
Example 2*: The Ruger BX-25 magazine, chambered for 22 LR calibre, is designed and manufactured for use in the Ruger SR22 rifle, the 10/22 family of rifles/carbines and the 22 Charger handgun. As a result, this magazine is a prohibited device unless modified so its capacity is 10 cartridges or less. (*This information was not included in the original version of this bulletin, but was added on 2013-09-05.)
What am I missing? Other than I'm not that smart.