9mm AR with a can - Edumacate me on barrel length

[PewPewPewCo]

On a blowback action, where is the velocity apex on 147gr 9mm? My guess would be somewhere around 10"

Not sure on he actual apex, but from 6" on its not gaining anything that's going to make a substantive difference, we're talking be difference of 20-60fps at any point from 6" on