I saw an example of this with my 300wsm. I had a 3 shot group with an ES 15 that printed with 2” vertical 4” over group at 600 yards. A higher charge I tested had an ES of 5 but the shot were spread out 13 14 inches verticaly. The velocities were quite a bit higher overall in that last group but not accurate despite good velocity. I saw this on a 6.5 saum once too. The tightest ES didn’t have the best group at distance. Was it the harmonics? And bad timing? Thanks for the thoughts.OBT was mentioned and that is all about barrel time. Accurate nodes occur at intervals of 100 - 200fps depending on barrel length. The shorter the barrel the closer the nodes become.
The 223 is a good candidate to look at. With a 24 inch bolt rifle the highest safe node is close to 3250fps with our propellants and a 55gr Vmax. Pressure calculates at 50K psi in QL. Reducing the barrel to 22 inch the charge needs to be upped and node speed is again 3250 - 60fps. Pressure now increases to 55K psi. For a 20 inch barrel the speed is still close to 3250fps and pressure now at 60K psi.
Shorter barrels are more rigid than long ones and that helps reducing the effect of barrel harmonics. Thicker tubes have the same effect.
Nodes and harmonics are two different animals and occur simultaneously. Node position we can calculate whereas harmonics can not be pinpointed easily. Nodes are just a name conjured up to explain the position of the sound pressure pulse that effects the diameter and shape of the barrel when oscillating from the action to the muzzle. Accuracy nodes are longest and easier to use when the wave is at the knox of the barrel and has the least influence on the muzzle shape and diameter.
Shorter half nodes are found central between the major nodes, but temperature changes can reduce or speed up the bullet and take it out of the band of the node. Thus the shorter the bandwidth is, the easier temps will influence groups.