It can fire both .223cal and 5.56mm, some AR's aren't rated for 5.56mm so be careful to anyone that owns one. Read the side of the receiver or if its a custom build be sure to read the barrel. 5.56mm cartridge generates far more pressure than the .223 round even though they are dimensionally the same.
The opposite is true of the 7.62mm and .308cal... The .308 is hotter than its 7.62 military equivalent. People with M1 Garands and M14's have reported bent or broken op rods when using the hotter .308 round, especially the light magnum rounds which will destroy your gun in as little as 100 rounds.
I prefer the 5.56 over the 300 black out or any other cartridge for that matter. The 5.56 round has come a long way since the 1:14 twist 55gr Vietnam days. The 62gr M855 penetrator round will penetrate armor @300 yards and fragment in soft tissue @ up to 400 yards. I prefer the 55gr M193 lead round for home defense and target practice due to its low chance of over penetration through dry wall and its economic price. The 7.62x39mm AK round is probably the most versatile round made in the world today. Its low cost and substantial power cant be denied. 1000 rounds cost under $200 and you can also hunt deer and similar size game much more effectively than the 5.56. I do prefer the 5.56 for varmints and smaller game like coyote due to its hyper velocity. The round hits before you even hear the gun go off. That's how fast the bullet travels in a 5.56mm...3250fps
