The barrel of a rifle has Harmonics, just like a car engine. Things that change these harmonics are things like barrel diameter, barrel length, weight of the projectile being spun, speed of the projectile being spun, the amount of interface between the bearing surface of the projectile and the lands of the barrel.
If you have spent any time shooting, I am sure you know to try different loaded rounds to see what shoots best in your rifle or pistol. What in affect you are doing by trying different factory loads or hand loading is trying to match the speed of the projectile, match the frequency node of the barrel. It was1939 that PWK invented the barrel tuner by adding a very simple weight to his barrel and the patent was granted in 1942 . Modern day Tuners are weights that you cut threads on your barrel to screw them on. As you might imagine, the harmonic frequencies involved are very high and thus change frequency in a very short amount of space. When you change the position of the weight, you change the frequency of the barrel, just like changing the length of the strings with the frets on a guitar. The harmonics of a gun barrel change and repeat themselves over and over with position changes to the weight. In only one revolution of the barrel Tuner on my Rifle, the whole thing starts over, just like the full musical scale with multiple "F" notes like F minor, etc. As you change the frequency of the Tuner, the group size will start to change and to find the node frequency of your barrel, you will shoot either 3 or five shot groups at each setting on the Tuner, till you get two or three settings where the groups stay the smallest and you split the difference and set the Tuner in the middle setting. Of course you will have to change the setting and re find the node, every single time that you change your ammo load. A different brand of ammo, you will have to change the tuner setting. A different weight projectile or different amount of powder or even a different lot of ammo and you will probably have to reset the tuner. but as long as the load stays the same, you do not change the tuner setting unless it starts shooting worse from say a big temperature change. This is a very finite adjustment such as for very long distance shooting but it does shrink your guns group sizes. I will attach some videos to watch.