Decibel levels 5.56 vs. 7.62x39

[BigBlue 94]

It's the bore volume. Both cartridges burn similar weights of powder, but the 7.62x39 has a much larger volume for that gas to expand into before the bullet clears the muzzle. So the muzzle pressure is, i believe, much lower for an equivalent length barrel.

Which would directly correlate to bore diameter. But of course powder type, would also play a role, depending on burn rate. .243 vs .308 is similar in that the 243 has a sharper crack. Same basic case, but a significantly smaller bore.

A 223 10" barrel has .39057 cubic inches of volume.
A 7.62 10" barrel has .74506 cubic inches of volume. Thats quite a difference. The same amount of gasses, when forced through each tube, will move faster as it travels down the smaller tube. This results in a sharper crack higher pressure, and higher velocity.

 

[Cannibul]

Did a search and couldn't find the data...
And...maybe this should be posted in the semi auto handgun section.
All else being equal
Say an AR platform with 10.5" barrel
Would the 5.56/223 around 3000 fps be louder than a 7.62x39 at approx 2200?
Logic tells me that once your above the speed of sound that there would be no significant difference.

3000 fps from a 10.5" barrel? Not even close.