It would have to be a double-stack mag for something like a TEC-9 or MAC-11 (9mm or 380), but the only way to know for sure is to actually try it in each of those; this appears to be an aftermarket magazine, so it will look different than the standard mag for those designs.
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Gun control in Canada: making the streets safer for rapists, muggers, and other violent criminals since 1936.
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