On the Canadian Firearms Safety Course, students are informed of the cartridge magazine capacity limitations which apply to Canadian firearms. These limits are generally five cartridges for semi-automatic rifles using centre-fire ammunition, and ten cartridges for handguns commonly available in Canada.
Rock River Arms is an Illinois firearms manufacturer which makes a very compact pistol version of the popular AR15 platform, which uses the ubiquitous .223 Remington cartridge (nearly identical to the 5.56 NATO). The LAR15 pistol is classed as a restricted firearm, and because it is designed to be fired with one hand, having just a 7" barrel and no buttstock, it qualifies to have the 10-round magazine that handguns are permitted in Canada.
The same magazine may be legally used in any other firearm that will accept it, including the popular AR variant semi-auto rifles available in Canada and which are normally limited to just five cartridges when using centre-fire ammunition.
That's right, because the magazine is designed to be used in a handgun it may hold up to 10 rounds but that same magazine may be legally used in any other firearm that it will fit into. Since the magazine is essentially a NATO standard, or "STANAG" magazine, it will fit almost any other AR style rifle.
The applicable regulations only require that the magazine be "designed and manufactured for" a particular firearm. In this case, the 5.56mm/.223 Remington LAR15 pistol, a handgun, and therefore limited to ten cartridges. The regulations do not proscribe using the magazine in another firearm that it was not "designed and manufactured for".
Another example where magazine limits can be effectively "doubled" from five to ten are the Beretta CX4 Storm carbine and the PX4 Storm pistol. As a restricted rifle, the 9mm Luger CX4 Storm ships with a magazine that is limited to just five cartridges. The magazine used is identical in every other respect to the ones used in the 9mm Luger PX4 Storm pistol. As the pistol enjoys the higher 10 round limit for its magazines, an owner of a CX4 carbine can simply use the pistol magazines in their rifle to double their capacity legally.
This unique situation also applies to certain rimfire cartridge magazines. Magazines made for handguns are normally limited to five (seldomly) or ten (more commonly) cartridges, as the case may be, regardless of whether they are for rimfire or centre-fire pistols. However, there is no limit for semiautomatic rimfire rifles. Magazines designed and made for rimfire rifles such as the Smith & Wesson M&P15-22 rifle, which come in capacities up to 25 rounds of .22LR, are capable of being used in the more compact close cousin, the M&P15-22 pistol.
And while this blog does not exist to give legal advice, and one should always carefully review the current laws and regulations with respect to firearms shooting activities, we are aided on the topic of magazine limits by a helpful bulletin published by the RCMP, Special Bulletin to Businesses No. 72. This helpfully interprets the regulations and their applicability to some of the more recent quirks of firearms and cartridge magazines that have come along since the laws - and the safety course material - were drafted.