Re: QuickLoad and Optimum Barrel Time?
Like bohem says, the keys are GIGO and understanding which QL setting to tweak, why, and when.
I've experimented with two rifles I'm shooting OBT-developed loads in, skipping between OBT-predicted accuracy nodes and watching what happens to accuracy. The answer was nothing. They shoot almost exactly the same MoA, to within less than 10%. And both those rifles shoot well under 1 MoA so 10% is a very, very small variation.
I started out using OBT to predict my starting load for OCW. The OCW testing kept getting more and more abbreviated until I finally just stopped using it. The OBT predicted charge almost always also was the OCW. Now I shoot a handful of rounds, just enough to get adequate data to tweak my QL settings, run one more small batch to confirm velocity (typically another three rounds), then I start tweaking seating depth.
Here's the notes on tweaking QL for OBT that were posted in another forum by Chris Long, who created OBT:
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My three main "tweaks" in QL are:
1.) Burn rate - no more than 5% higher or lower. More than that and I go to #2
2.) Weighting factor - no more than 10% change from QL default, else go to #3
3.) Bullet weight - this is a very good way to compensate for bore friction and other physical things that QL can't model. An increase in friction effectively reduces the accelerating force on the bullet, slowing it down. Increasing the bullet mass does the same thing in QL (F=MA). If you have to tweak this by more than a few single digit percentages, then there is something very weird going on. If things get too weird, reset it all to QL defaults, and start over with just the bullet weight and see what you get. I had one Savage 6mm barrel on a friends rifle that was really rough, and required a 3 grain increase in bullet weight (from 105 grains) to get the velocities to match. It was the only tweak that would let the model even get close to the right predictions.
The process is trial-and-error, tweaking these parameters until you can match at least two velocity measurements. I usually fire at least three different charge weights, from low to high, trying to bracket the expected OBT velocity range(s), and record velocity for each trial. It is best to fire 5 shots at each weight, if you have the patience, as the velocity average will be much more accurate than a single shot value. Single shots are OK to get you in the ballpark, but to get a good QL model, you need more than 1 at each charge weight. Then, tweak QL until you get the best match to your data samples.
You will find that it is very difficult, if not impossible to get an exact match for some component combinations. I am convinced that there are non-linear effects in the actual powder burn process that QL's linear system models do not encompass. However, if you work at it, you should get the model to match well over the range of velocity that encompasses the OBT you are shooting for. If in doubt, try narrowing the test velocity range around the target OBT node, and try again. Once you get QL calibrated, you can adjust the charge weight in QL to hit an OBT on the nose. That will be your starting load for an OCW type test.
