Here is some calculated numbers using Quickload:
Bullet: Hornady 124gr FMJ RN
Powder: Unique 5.0grains
Calulated max pressure: 31.5kpsi with full burn in a 4" barrel.
4" barrel: 1141fps
5" barrel: 1196fps
6" barrel: 1238fps
7" barrel: 1270fps
8" barrel: 1296fps
So it seems that there are indeed more power in a 9mm from a longer barrel.
As you can see the gain for each inch is decreasing.
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