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they are available from the factory (MPA) here https://masterpiecearms.com/shop/5-7x28-20-round-magazine/ it will work with both the 20 and 30 round FN 5.57 mags

here is a Pro mag 30 round mag https://www.budsgunshop.com/catalog/product_info.php/products_id/77665/promag+fnh-a2+fn+fiveseven+usg+magazine+30rd+57x28mm+black+polymer with promag I suggest trying one before buying more

The new design also provides complete compatibility with the FN57 magazines in both 20 and 30 round capacity. so any 20 or 30 round mag here should work https://www.budsgunshop.com/catalog/search.php?q=5.7&type=Magazines

Travis, Buds has magazines available here : https://www.budsgunshop.com/catalog/product_info.php/products_id/85467/masterpiece+mpa5770+def+57x28+57mmx28mm%2fcal+20rd+blk+finis ------------ https://www.budsgunshop.com/catalog/product_info.php/products_id/17123/fn+herstal+20+round+57mm+x+28mm+magazine+w%2fblue+finish ----------- https://www.budsgunshop.com/catalog/product_info.php/products_id/77665/promag+fnh-a2+fn+fiveseven+usg+magazine+30rd+57x28mm+black+polymer