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If installing it on one of CMMG's Mk57 lower receivers it will use FN Five seveN magazines. On a regular AR lower receiver you would need the conversion magazines like this: https://www.budsgunshop.com/product_info.php/products_id/713091798/cmmg+magazine+57x28mm+ar15

Shannon, AR Style 5.7 Magazines are needed : https://www.budsgunshop.com/product_info.php/products_id/713091858/cmmg+magazine+57x28mm+ar15