Noise of .223/5.56X45 and 7.62X39?

The 5.56 in my opinion is a bit louder than the 7.62x39. It's a more solid and sharp "CRACK". Either way, both are loud shells, and it really gets to a point where you don't notice the difference between them (if you are at a shooting range all day, then you aren't really going to notice). The 7.62x51 is a bit louder than the 5.56, since it is a larger cartridge, utilizing more powder and consequently produces a larger volume of gas. Again, not a huge difference between them, but if you listen carefully you will see.