I have been listening to the news on this and most everyone has said it is a .223 nearly the same as a 5.56 mm. Most people would not be able to tell the difference by looking at them.

The .223 is just almost the same diameter as the .22. The .223 is 223 thousandths of an inch in diameter. The .22 is 220 thousandths of an inch in diameter. That makes the .223 three thousandths of an inch greater in diameter. Essentially, the 5.56 mm is the same diameter but it is built to metric (NATO) standards while the .223 uses the American measurement of caliber or hundredths/thousandths of an inch.

Until Vietnam, the US military used 7.62 mm or .308. They went to the M-16 that fires the .223 for cost, weight and ease of shooting. The .223 does not kick nearly as much as the .308. I know. I shot both. The .223 I could shoot all day. The M-14, 7.62 did a number on me. By the time I fired it for qualification, my lip was all swollen up for about a week after that.

However, it is not all about the bullet diameter. The .223 is longer and is built in such a way that it is much faster and deadlier. The cartridge, or that part that holds the gun powder is considerably bigger in diameter. It is what allows it to fire at speeds of over 3000 ft per second. Indeed, the M-14 can kill at over a thousand yards in the hands of the right person, even without a scope. I’m not so sure about the m-16. Not only does it lack the range of the M-14 but it is less accurate to begin with.

That being said, the gun that the shooter used fired the .223, a variant of the AR-15, which is something of the civilian version of the M-16. However, not all AR-15s use .223. They can be purchased to fire larger and smaller rounds. I have seen a few that fire the 7.62, or the M-14 round. Not many buy them for a number of reasons, mostly cost of ammo.

This is the very abbreviated version. It is easy to look it all up on the internet. On the other hand, you can go to a place that sells guns and they will likely be happy to explain it, or maybe even show you. There is simply no excuse to talk about all this on the news without knowing all this when it is so easy to look up.