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Hi Luther, it will run, but I haven't personally tried it, so I can't say how well. Here is a dedicated 300 blackout magazine https://www.budsgunshop.com/product_info.php/products_id/133215/magpul+mag1182-blk+pmag+gen+m3+300+blackout+20rd+black+polymer+detachable , Stay safe & have a great day!

It is supposed to.

Hi Luther and welcome. It runs 100% in my 300BLK Upper. No worries. Cheers, stay safe and enjoy!