This explains it in detail.
https://en.wikipedia.org/wiki/5.56%C3%9745mm_NATO#5.56mm_NATO_versus_.223_Remington
The 5.56mm NATO and .223 Remington cartridges and chamberings are similar but not identical.[45] While the cartridges are identical other than powder load, the chamber leade, i.e. the area where the rifling begins, is cut to a sharper angle on some .223 commercial chambers. Because of this, a cartridge loaded to generate 5.56mm pressures in a 5.56mm chamber may develop pressures that exceed SAAMI limits when fired from a short-leade .223 Remington chamber.