Converting your 5.56 NATO AR to 9mm is a really simple AR hack, especially if you buy a complete 9mm upper.
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Simple AR Caliber Change: EndoMag Insert Converts Your Lower to 9mm
Updated: May 26, 2019Pistol Caliber Carbines have earned a place in your gun safe. Fun, inexpensive and easy to shoot, they even have their own competitions. The Mean Arms EndoMag magazine insert converts your Gen 2 or Gen 3 5.56/.223 PMAG® housing into a 9mm magazine and conversion block mating a standard AR lower to your 9mm upper. This idea is simple, effective and brilliant.