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Yes, that will work just fine. 300 blackout and 556 use the same lower, buffer tube and everything. Only thing you might need to change is the buffer weight but shouldn't need to. Hope this helps.

Jeffrey, yes Sir. No issue with fit and function with a PSA 300 aac Rifle upper. 300 AAC and 5.56/223 work on the same AR platform. Cheers and stay safe!

Yes this is a multi Cal lower and will work without issue with a 300 blackout, most 556 and 300 blackout parts are interchangeable (except the barrels)

Yes you can

Jefferey, yes you can use PSA 3900AAC upper with this lower. Have a great day!