Again I am ignorant, I am weary of snake oil.
I do not want mean to sound condescending but I am just trying to figure this out.
I have followed Eric for some time now on YouTube and I have enjoyed his input but...
It's fine to be skeptical, but it is proven science. A barrel is just a tube. Like any other tube or pipe, sound resonates thru it. The length, shape, thickness, and material it is made from, all contribute to determining the natural frequency the barrel will resonate at.
When you fire a shot, this resonating actually begins before the bullet has left the barrel. Just like a cattle whip, this causes the end of the barrel to "whip" back and forth. Tunning a barrel is simply trying to control the vibration so that the "whip" is reduced or the timing is matched to the bullet leaving the rifling.
The Ruger Mini 14 is an example of a rifle that suffered from poor harmonics. The new ones are supposedly improved, but prior to this decade, the Mini 14 was known to be 2- 3 moa gun, if you were lucky. They came out with an accessory called the " Accu-strut" that did help improve accuracy by changing the barrel harmonics and stiffening things up, but it still didn't make it a precision rifle.
A tuner is not a magic fix it. If your rifle will not shoot moa, a tuner may or may not help it get there, depending on how bad it is.
If you have a rifle that's already shooting sub Moa, unless you are a pro like Eric Cortina, you are not likely to see the difference adding a tuner may make. It may trim your groups from 1 moa to half moa, but can you shoot good enough to see that difference?
If not, then all a tuner is gonna do is give you something else to play with and add another variable to your load development. After you have control of YOU and all the other variables in your loading, then a tuner may help.