I bought a few at the York show as they were cheap but the bigger question is why? I thought one of the selling points of .300 Blackout was that it ran on a standard AR bolt and reliably with standard AR mags?
If they are labelled ".300 Blackout" in easy to see writing, then it's likely to help keep you from putting a mag of Blackout in a 5.56 rifle. If not, then you fell for the PT Barnum Principle.