A friend of mine wanted to see how far he could go with .308 Win - figuratively and physically. He had several barrels made with twist rates as tight as 1:3.5".
But he never got there. He found 1:5" to be unnecessarily fast and overall detrimental - to the point that even gain twist barrels were peeling jackets and ruining bullets.
In rimfire world, a lot of e-ink is spilled arguing about barrel length. He thinks 22 inch and longer barrels allow the pressure in the barrel enough that the bullet comes back to its normal shape a bit after having been pushed a bit wide at the base during earlier acceleration.
I noticed that as well, and quietly shook my head.
We have decades of .22 LR testing that clearly shows longer barrels reducing ES and SD, often theorized to be from pressure normalization and pressure curve 'smoothing'.
Regardless of one's thoughts on why ES and SD improve, it is bewildering that someone can jump to, "it lets a deformed bullet get back into shape."
--
It isn't really the same topic, but one thing that I do know about our barrels is that I have to watch muzzle velocity. My 22" barrel pushes a few 'subsonic' and 'standard velocity' loads into supersonic territory - including TAC22 (that might be part of its issue). However, my son's 16.5" barrel produces velocities that are typically just shy of advertised numbers.
He can shoot anything subsonic (so far) and it will stay subsonic.
But I have to be careful, especially in cooler weather.
He has to deal with lower velocity and more drop, across the board. But I have to worry about trans-sonic destabilization, right out of the gate. Which is, arguably, the worst place for a bullet to go trans-sonic. (Destabilizing something that is already in the zone of initial stabilization.)
Last time I was testing, CCI Standard Velocity was not shooting as well as it normally does for me. I shot some across the chronograph and found it to be 'perfectly' straddling the trans-sonic zone at 5 feet. Speed of sound at that time was 1104 fps, which was exactly my average.
But, as we get warmer weather, I expect temperature-driven velocity increase to be lower than the rate of increase for the speed of sound. So, it should be safe until Sept/Oct.
The rifle shoots some other loads as well, or better, but nearly all of them are 5-10 times the price; or they are 'high velocity' loads and put the transition zone between 50 and 80 yards. I don't have deep enough pockets for the expensive ammo, or a desire to go trans-sonic at a range that creates potential issues for 50% of my targets.
Sorry, that turned into a bit of rambling. I got thinking and some of it leaked out of my fingertips.