There's a couple misconceptions and I'll try to bring them to light. The .223 is not a commercial eq to the 5.56 since it is and has been for some time available to anyone. do not EVER EVER, NEVER fire a 5.56 in a .223 gun. Here's the only differences and I'm going to try to make this simple since most people don't really understand the principles of loading. 1. The 5.56 cartridge has a thicker wall than the .223 because of the difference in pressures created INSIDE the Chamber of the weapon, has nothing to do with internal cartridge pressure. 2. The Throat or what's called the leade is slightly different on the 5.56 and that's basically the distance the bullet travels after ignition into the rifled part of the barrel. Since the 5.56 is just a tad bit longer. Basically, throw everything you've ever heard about the difference between the two out of the window, the two are not the same. They aren't even manufactured the same way, there's a complete different step that has to be taken when loading a 5.56. Ever notice that slight discoloration in the neck of a 5.56? that's due to what's called "flashing". and without this step, upon ignition, the thing would just blow up. If you are building an AR, go with the 5.56 bbl and lower, that way you can shoot either and not have to worry.