BigBlue 94
Member
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.