I'll take a shot at what harmonic tuning means to me. (This is from the rimfire point of view, weight on the end of the barrel and micrometer adjustments).
What I was told the barrel acts like a tuning fork. Constant up and down motion. You want the bullet to exit when it's in the center. With rimfire ammo your set to a constant velocity. For the ease of discussion lets say my gun shot X ammo at 1050fps, but not great. Since you can't vary the velocity from the powdered stand point with the tuner you can adjust the barrel harmonics. Basically with the tuner your changing the barrel length to get the barrel in the proper position. (Center of the wave).
In air rifles I think it should only be a matter of varying the velocity a few feet per second. Till you reach this node. I might be all wet on this but I thought that was one of the plus points of an Impact the Micrometer bumper adjustment?
Side note, the liner indexing has me curious also. I really can't see how this will help with harmonics but I can see where it will change how the projectile leaves the barrel.
If you get right down to it. Some other things to look at is liner tension. How tight you pull the barrel from front to back. Then there's the oring placement it in your barrel (so to speak) if it is vibrating.
Great thread if you can't tell I've thinking about this some.