Will a 300 Blackout round chamber in a 5.56 barrel?

It will chamber. That is exactly what makes this the most dangerous mix-up in the AR-15 world: a 300 Blackout cartridge is built on a shortened 5.56 case, so it drops into a 5.56 chamber and the bolt closes on it with nothing to warn you.

What happens next is the problem. The 300 Blackout bullet is .308 inches across, and a 5.56 barrel has a .224 groove diameter. On firing, the bullet has nowhere to go — it is driven into a bore far too small for it, pressure spikes far past what the barrel and receiver were built for, and the rifle can come apart. Rifle makers publish standalone warnings about this specific mix-up, which is not something they do lightly.

On the formal listing, be precise about what it says: SAAMI names .300 AAC Blackout among the cartridges not to be used in firearms chambered for .223 Remington. It does not list it under 5.56x45mm NATO, whose entry names only .25-45 Sharps. That is a cataloguing distinction rather than a reprieve — a 5.56 barrel has the same .224 groove diameter the hazard depends on — but the citation is worth quoting accurately rather than stretching.

The practical defence is separation, not vigilance. Keep the two calibers in different magazines, never have both out on the bench at once, mark 300 Blackout magazines so they are obvious by feel, and check the barrel marking before loading a rifle you have not shot that day. This almost always happens to people who own both and were being careful.