Attempting to fire 5.56x45mm NATO ammunition in a firearm chambered for .300 AAC Blackout can lead to catastrophic firearm failure and potentially serious injury. While the two cartridges appear similar in size, their dimensions are critically different. The .300 Blackout uses a larger diameter bullet and operates at significantly lower pressures than the 5.56x45mm NATO round. Firing the higher-pressure 5.56x45mm round in a .300 Blackout chamber can cause the barrel to burst, the bolt to fail, or other severe malfunctions.
Understanding the incompatibility of these two cartridges is paramount for safe firearm operation. Confusion can arise due to their similar case dimensions and the fact that both can be used in the AR-15 platform. This makes proper cartridge identification essential. The consequences of misidentification can range from firearm damage to severe injury or even death. Promoting clear education about this critical difference is fundamental to responsible firearm ownership and usage.