Experiment with OBT and Quickload Data

[CyaN1de]

So I spent last night making up some loads for both a ladder test and to test out the OBT (Optimum Barrel Time) theory today.

I have been wondering how accurate the quickload data would be for velocities vs powder charge etc.

When I got home from the range and cross referenced my real world data with the Quickload Data I was pleasantly surprised. (Results Below)

I started with the OBT file to find out which of the barrel times I could safely load for. It turned out that the middle time would be a very Hot load at 31.4 grains of Varget in my 6BR and I was not comfortable loading them to this capacity. So I opted for the next slower time which is quite a lower charge than I would have liked but was a safe load of 28.7 grains.

OBT

Quickload suggested the 28.7 grain load based on the shown OBT with a velocity of 2684fps.

This load grouped well at 200m, not stellar, but decent for the barrel I am using for this experiment. (1/3 - 1/2 min.)

The real surprise was when I downloaded the chrony data. I had shot 2 - 5 shot strings at separate times during the day today.

Velocities came within 4fps of the velocity that QL had predicted.
Avg of the 10 shots is 2686.6

The only tweaks I had made to Quickload was to change the start pressure to 5000 because I was jamming the bullets .015 into the lands.

Conclusion:
I think there is some merit for this OBT theory. Match it up with Quickload for a very accurate combination which could save someone some powder and bullets in the load work up area. Although every rifle will differ, I think this is a good starting point for load development and will try it again with different rifles/calibers/barrels in the future.

 

[martinbns]

Wow, that's very technical.

Interesting how close it was, I've had mixed results with comparing actual velocities to projected QL ones. The only one I'e really compared was a 264 win mag that varied almost 200 fps, real world was faster.

 

[acrashb]

I have found quickload to be remarkably accurate when fed accurate info for bottleneck cartridges. The most important part being measuring your actual case capacity from a fired case with the water method.

 

[kombayotch]

I've been doing that for a while with QL. It works well if the QL data for that powder is accurate. I've had to fiddle with the Ba value on some powders to get things to match. That means however, that the pressure numbers are no longer valid. Not an issue since QL shouldn't change the way you work up a load.

You probably will want to verify that the velocities also match for other powder charges as well.

 

[CyaN1de]

Part of the experiment was to see how close QL was to real world numbers and to take the real world data and tweak QL if necessary.

Apparently for the load data I used it was almost bang on.

Of course I would check QL with real numbers on other cases/loads/calibers before putting my faith in QL as a starting point.

 

[kombayotch]

Not what I meant. It appears that you only compared 28.7 grains to the QL results. What I was saying was that you need to compare different powder charges also and see if those line up as well.

What you're trying to do is match two curves (both Powder Charge vs. Muzzle Velocity). You're trying to match a theoretical/calculated curve (QL calculations) and a measured curve (range results). Your doing this to derive a value from the theoretical model (barrel time) because you cannot measure it easily or derive it from your measured results. In order for this to be valid, you have to show that the two curves superimpose each other. You have only shown that they intersect at one point. You need more than one point for any kind of data correlation to be valid. This is a fundamental scientific principal.

Its easy to get QL data to match the velocity of a single charge weight by "adjusting" various values. However, if the results do not match for other charge weights as well, your barrel time number is not accurate.

When I want to determine an OBT time for a given load, I work up the load in the usual way until I find pressure (without any thought of OBT). Then I verify that the velocities QL gives me match the results at ALL charge weights. If it doesn't, I adjust things to get the numbers correct across the board. Then, I calculate the OBT once the software results have been correlated to the real world results.

 

[kombayotch]

Even doing this, without matching pressure as well, the results may not be accurate. To verify that you are at the OCW, you need to load some rounds at 28.4, 28.7 and 30.0 gr. and verify that they all impact at the same location at a reasonable distance like 300 yards.