I'm a competitive smallbore rifle, precision air rifle, sporter air rifle, and air pistol shooter, and the current American Legion national champion in Sporter class air rifle.
There is no point in testing a barrel in a vise. In a vise or fixture eliminates barrel harmonics, that is what your
really testing for. When a shot is shot, the moment the bullet leaves the case the barrel starts to vibrate. Mostly up and down, and the point you want the bullet to leave the barrel is when the barrel is in the topmost or bottommost position of it's harmonic. Because there is a little pause at the top and bottom of the wave, that is the best place to have the bullet exit the barrel for utmost consistency. When you have a barrel/load combo that exits the barrel when the barrel is not at the top of bottom most position, it can hurt accuracy and in a competitive situation a barrel tuner should be used. Check out Beesting tuners if your interested.
All this happens in just a few miniseconds, faster than the eye can see. But it's there, and even less on airguns. But it's still there and has to be accounted for.
https://www.youtube.com/watch?v=AqgPu9gSmVc