CharlieNC, I was going to disagree because I was certain Chris had neglected to specify that detail, but I HATE the taste of feet so I went back through his stuff first to reduce the chances I might have to stick mine in my mouth and, SHAZZAM, and you are right. My apologies to Chris Long for besmirching his work. From the
Optimal Barrel Time Paper:
I must admit I had overlooked that detail. Or forgotten. Most likely overlooked.
ShtrRdy, one thing to consider is that OBT doesn't claim to be able to determine the location of
all accuracy nodes, only those nodes created by a specific harmonic node, the one Chris theorizes to be most influential to accuracy. It does not rule out the existence of other accuracy nodes. One of its drawbacks is that if you use OBT to full effect, use it to make your load development process as abbreviated as possible, you necessarily are leaving other, possibly more accurate combinations untested.
That said, I'm not sure what you were hoping to discover, but I don't think your data supports your conclusion. Since you arrived at that original OBT charge weight by your own tweaking, the fact that other charge weights grouped tighter does nothing to indicate whether the problem was in your tweaking, or in your choice of barrel times, or in something altogether different.
In any case, a charge weight increment of 0.7 grains can only move you from one OBT node to the next adjacent node in the case of a light bullet in a low capacity cartridge fired through a short barrel. I just checked my pet .204 load in QL and doubling that charge amount to 1.4 grains
will get me onto the next node, exactly, but only in the case of an already compressed load, and only then if I dial the barrel length back to 16", which moves the nodes closer together. So unless you're shooting a 45-gr bullet from a 14 1/2" M4, I fail to see the practical application.
The objective of the initial OBT range sessions has nothing to do with OBT
per se and everything to do with truing your QL settings to your range data. It takes at least two points of data to establish a trend, and the greater the distance between them, the more validity to the trend. Which means you have to load at least two different charge weights, as widely separated as practicable, and then tweak QL's parameters to make the data from
both loads fit. From your description I don't get the sense that that's what you've done.
There is no reason you couldn't begin trying to match data at two points on your first range session, but I liken that to trying to build a bridge by starting in the middle of the river and working toward either bank. I don't bother with a second charge weight until I finally get QL and the chrono to agree on the first load -- which sometimes takes as many as three range sessions -- because up until then, none of the QL settings has proved itself. At least when QL and the chrono finally agree on one load, I know I have one valid point of data.
But only one. And one is not enough. One point you can match by pure coincidence. The more points you match, the more the Principle of
Occam's Razor rules out coincidence. So you might be guessing at the QL settings, but the more points you can match, the greater the likelihood that your guesses are causing QL to create a simulation that accurately represents a real world phenomenon.
Which also is why there are multiple properties available for tweaking. Tweaking for one load is no challenge whatsoever. If that was all there was to it, Ba is all you'd ever need. But you might find that increasing Ba moves two loads closer together for a while, but then they cease converging. Or you reach Pmax. So you leave Ba where they came closest, then try tweaking weighting factor. Or bullet weight.
Tweak QL, predict new charge weight, load, test, repeat. Until QL and chrono agree. The process is a converging spiral, and it takes as long as it takes before you hit dead center. Only then do I introduce the additional complication of the second charge weight. If its MV doesn't match QL's prediction (anything else is a rare as hobbyhorse shit), then I begin the complex process of arbitrarily tweaking whatever QL property trips my trigger, but always aware that these fudged settings still have to match the MVs from
BOTH loads.
Then I turn to finding the optimal seating depth. I typically fire more rounds tweaking seating depth than finding the OBT charge weight, at least when it's the first time I've tested a particular bullet. But it's not the charge weight that's magic, it's the MV, because the MV is the indicator of OBT. So that charge weight
might change when tweaking seating depth. In my .308 loads, for instance, each change of 0.01 in seating depth roughly correlates to 0.1 gr in charge weight. So I tend to start seated to max magazine length, then seat deeper, reducing charge weight as necessary to maintain that magic MV.
So just because you've got your OBT node's MV nailed, you've still not made optimal use of it, not exploited it's full accuracy potential, until you've found the optimal jump that also maintains the magic MV.
