It's not a myth. Velocity does change with barrel length. How much depends on the ammunition, caliber and gun.
Here is an example. My granddaughter did a science project showing something else but she needed to know what changes to velocity occurred.
She started out with a 26 inch 308 barrel, and Remington factory 180 grn corlok bullets.
The first column is barrel length the second is velocity (at 10 ft from muzzle.)
Quote:
26 2826
24 2722
22 2713
20 2677
18 2654
16 2552
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