There are a few important differences between 5.56 NATO & .223 Remington, but the biggest issue will be pressure. 5.56x45mm NATO is loaded to higher pressures, and 5.56 chambers are slightly longer. If you put a 5.56 cartridge in a .223 Remington chamber, you could create a dangerous pressure spike because of that reduced lead and hotter loading. You SHOULD NOT use a 5.56x45mm NATO cartridge in a chamber marked for .223 Remington. However, you CAN use .223 Remington in a 5.56 chamber, so if you're building a rifle up from scratch, you'll want a chamber made for 5.56x45mm NATO. Alternatively, you can choose .223 Wylde, a hybridized chambering that safely accepts both 5.56 and .223 with exceptional performance.