...There are 5 separate and discrete ideas floating around in this thread...
Was just about to say this, more or less.
One is that as you increase charge weight, you will see a "flat spot" in a chart of charge weight vs velocity. I have never seen this in way that convinced me that it was anything other than random. If you shoot 10 rounds, you're very likely to see a flat spot even if the highs and lows are random. Others swear they're there. I'm not sure what you'd do with that information, but I've never messed around with it, because I don't see it in my rifles.
Another is what al is calling "sine wave tuning". This is undoubtedly, take it to the bank, real. It's been documented thoroughly. Some call it positive compensation. If you shoot one shot, each one inch the the right of the last in increasing charge weights, you would see a stair step pattern - IF your rifle is accurate enough. Then, if you correct those points for variation in velocity drop, you'll find al's sine wave. The reason for this is that the weight distribution of the rifle is in the up/down direction - the scope is on top of the rifle, and the stock is below it. The recoil drives teh rifle in an up down pattern because of the inertia of the rifle above and below the bore. That's why the target pattern is up and down and not left and right.
Then you get to generic load development. If you see the sine wave pattern described above, you might come to the conclusion that that is the only thing going on -that the barrel is whipping about in a sine wavey pattern up and down. But it's not. In reality, that is just one mode of vibration, albeit a dominant one. If you shoot enough groups, you might even find some that are horizontally bad, but vertically good. That's because there are other modes in there too - at different frequencies that only show up on the target under certain conditions. Load development as we know it is finding a combination that minimizes group size for all of the vibration modes at the same time. With very accurate rifles (PPC, etc), this is clearer than with less accurate rilfes, but you can see it if you know what to look for. I'll call this "tune".
However, tune is not all there is to accuracy. It's very roughly only half the battle. Bullet quality is another major, if not dominant, factor. Thankfully most of the bullets we use are of very high quality - jacket wall thickness tolerances are obscenely small. They're very straight. Weights are consistent. But if any of those are off, your "tune" doesn't mean squat - the bullet WILL fly off in the wrong, unpredictable direction. We don't think about this much, because it's basically a solved problem - use good bullets. Twist rate matters as well. There's a reason that my .30 BR with an 18 inch twist is more accurate than my .308 wiht a 10 inch twist. Bullet length (I'm simplifying here) matters - this is another reason my .30BR is more accurate, and it's also why I can get away with the helpful 18 inch twist. For some reason people resist this idea even though it too has been thought through and carefully documented. Bryan Litz's book doesn't help, but it's not really his fault that people are misinterpreting his writing.
And finally, I wish we'd drop the word "node" from our vocabulary. It's just not a useful term, and in no way does the node on a standing wave correlate to any of these concepts. It implies certain things that may or may not be true, and it's just confusing to people who don't know that it's shorthand for something else.
