"The tuner is a weight which can be precisely position by minute steps, allowing you to find the “Sweet Spot” of the tuner, by precisely adjusting the position of the tuning weight, until the barrel oscillations, create “positive compensation” this greatly improving the accuracy with ammo of higher or lower velocity. This avoids vertical groups on the target.
Being able to “tune the barrel” is an art long since studied by long distance bench rest shooters, but largely ignored by many Olympic Rimfire competitors. This is a shame, as the tuning can make a real and meaningful improvement to your rifle’s accuracy!
As a bullet speeds through the barrel it creates a whipping effect in the barrel, much like a Sinus wave form. This is minute of course, but it does mean that a faster and a lower bullet will launch from the crown of the barrel at a slightly different angle within that sinus wave form. This leads to a vertical displacement on the target. Bearing in mind that even with good ammo, the variance in speed can be as much as 30 fps – this is a big deal! Usually, the faster bullet will hit higher on the target, and a variance of 30 fps will translate to about 5mm height difference on a 50m target. This is too much! Way too much!
In order to compensate for the variation in velocity, we need to verify that both slow and fast bullets will exit the barrel on the upper slope of the cycle, so that slower rounds, which stay longer in the barrel, will exit it on a higher angle than faster rounds, thus compensating for the difference in velocities, and avoiding vertical dispersion. This is called “positive compensation”.
There are many tuners on the market - most in the $300CAN range (Harrell's, Starik, Ezell etc.)."