Hi David and welcome back. I have several Radical Rifles and several of these magazines and 5.56/.223 runs 100% and 300BLK runs 100%. Some reliable dedicated 300BLK 30rd magazines here: budsgunshop.com/product_info.php/products_id/114333/magpul+pmag+m3+300black+30rd+black --------------------------------------------------------------- budsgunshop.com/product_info.php/products_id/723019148/black+rain+ordnance+lasered+ar15+magazine+.300+black+30+rd. -------------------------------------------------------------- budsgunshop.com/product_info.php/products_id/713098272/d%26h+tactical+magazine+.300blk ----------- Cheers and stay safe!

It's a very common question and one that is easily answered. .300 Blackout will fit in a 5.56 magazine without issue.

Good afternoon to you!! Yes a 556 magazine will feed 300 blackout. The 300blk uses a 556 cartridge shortened and necked up to 30 caliber. They will feed no problem. Hope this helps. Have a good rest of your day.
🇺🇸🇺🇸 'Merica' 🇺🇸🇺🇸

Yes.

Yes they are compatible. .300 Blackout and 5.56 use the same platform: same bolt, magazine etc., but require different barrels. Ar15 magazines do indeed work for .300 blackout.