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The main thing is barrel length. A longer barrel allows the gases to loose energy. Handguns excel in turning powder into sound and rifles put it more into the bullet. For example the 223 has more bullet muzzle energy than a 44 mag, but the 44 mag makes about 8 times the peak sound energy. I think that 9mm and 40 will be pretty close given the same barrel length.
The main thing is barrel length. A longer barrel allows the gases to loose energy. Handguns excel in turning powder into sound and rifles put it more into the bullet. For example the 223 has more bullet muzzle energy than a 44 mag, but the 44 mag makes about 8 times the peak sound energy.
I think that 9mm and 40 will be pretty close given the same barrel length.
