Rod- any time you change the harmonics of the barrel, you change the 'time' of the bullet's departure to a different muzzle location, IE: the change of harmonics, as I understand this, is to the wave length and maybe shape as well - so- in stead of the bullet exiting at a low node, or high as the case should be, it exits somewhere else, thus a change in the POI.
; The browning brake, adjusts the weight in or out, only slightly, but it has a profound effect in some guns, expecially with light barrels.
: Buddy of mine bought a .338 Browning from a deceased friend's widow. The original owner thought it shot just fine, and it did, for a moose hunting rifle - about 2" with his loads(which was the same load used by buddy-normally. The original owner always thought his 2 1/2" groups were due to him, not the rifle.
: Buddy adjusted the brake about 3/8" in all, and the same group shoots into .6" for 5 shots. Barrel harmonics are an interesting thing.
: I've known on match shooters who pick a load due to it's consistancy, then cut 1/8" off the end of the barrel until the load shoots sub 1/2", then leave it there. it usually only takes an inch in all of change to get the harmonics to exit the bullet at the top, or bottom of a node. Big groups come from consistant Sd loaded bullets exiting in the middle of the nodes or from poor Sd's loads tht exit all over the specturm from low to high. These usually shot a low of vertical in the group.
; I don't have much of a handle on barrel harmonics, but this is some of what I've learned - right or wrong- seems to work in thoery.