In doing more online research into this topic, I looked at Nordic Marksman website for the bloop tubes with tuners, and what caught my eye was the Guy Starik Carbon Tube barrel tuner....., and one that fits my rifle just so happens to be in stock, and in red and black (which would look very nice on my rifle!). Oh, do I want to try out this bloop tube tuner! (But I have to hold back a purchase for now due to budget limitations).
So I looked up Guy Starik's website, and watched his two videos on the tuner.
https://www.starikshooting.com/
The website describes the theory. The videos are short duration (3 to 3.5 min), and well done, informative, well worth your time to watch if you have never seen these, and especially worth your while if you are a skeptic.
He uses a proper laboratory testing facility with rifle clamp, shooting tunnel, and an electronic POI sensor and computerized group dispersion measurement system.
In video #2 linked below, he shows the tuner being used to decrease group vertical dispersion of two lots of Eley Tenex ammo that have been tested to be 30 fps muzzle velocity difference. Normally this 30 fps velocity difference would result in vertical dispersion of about 5mm at 50m, which does show on the measurement screen. On his website he posts a data example showing 0.24" vertical dispersion at 50m between rounds going 1035 and 1075.
As we all know, a 30 fps difference, round to round in a single lot of mid-grade target ammo is typical. And removing all other variables, this accounts for how difficult it can be to acheive the 1/2 inch group challenge, and how the ammo can lose you a match.
In his tests, he claims that his tuner can create "positive compensation" to improve the group's precision of rounds with a different velocity up to about 30 fps difference, thus reducing vertical dispersion.
Caveat: I am well aware that he is selling a product. As a science professional, I "believe" nothing, and I keep an open mind. I look for repeatable experimental evidence, control of variables, Type 1 and Type 2 errors, etc.
Video 2 about how the tuner works, and showing his electronic testing facilities. He suggests shooting 10-shot groups using 5 rounds each of the two different known velocities, and then using the tuner to reduce the vertical dispersion. I did not hear him say that this works every time for any ammo, for any rifle, nor does it say that in text on his website. So integrity-wise, I give him credit that he is offering you a high quality equipment option, and then its up to you to determine how to use it and to see if it works for you.
Interestingly, in both videos he is also using a rubber deresonator dampener weight ("tuner") part way down his barrel. I use a Limbsaver Deresonator on some of my rifles, and I "think" they make a difference, but there is no way I can "know" without being able to test it scientifically in a proper shooting lab.