FYI, this happens around 12-18" for 9mm, depending on the exact load.
Now, velocity gain past 6-8" is pretty much nil. You literally get only about 50fps going from 8" to 16" with most 9mm ammo.
And yes, 45 is similar. It's actually even more tapped-out by 8" in most cases, due to the low pressures and large bore.
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