Experimental proof that LC09 brass weight is correlated to case volume.

[jlow]

Donovan - I’ve looked at your setup and it is impressive as are the numbers, but to me comparing ES/SDEV only makes sense when we are talking about the same type of gun and intended use. Again, an AR-15 is not a BR gun and vice versa. An AR-15 will never have the accuracy and reproducibility of a BR gun but it was not build to be. Conversely if you have to lay down suppressive fire on bad guys, the BR gun is next to useless. But that does not mean that the AR-15 or BR gun is deficient. The old adage of use the right tool for the right job applies here. I am not familiar with the 17-HMR but again if you want to compare apples to apples, take a quick look at what the ES for a XM-193 223 round would be out of an AR barrel. I can tell you it is closer to 100 fps.

So let’s talk about the ES. The max range for a 223/556 AR-15 is about 600 yards. So my average MV is 2738 fps and I have an ES of 30 fps. If you plug the numbers into a ballistic computer and see what is the significance of this “horrible” ES? Taking a low number of 2708 fps for an instance. The fact is the elevation drop of 2738 fps under the conditions of my shoot today is 16.50 MOA at 600 yards and a slower bullet i.e. 2708 fps is 16.75 MOA. So that is a difference of 0.25 MOA which at 600 yards is equal to 1.5”. The X-ring on an F-class target is about 3 inches and so could the ES be better, sure, but it seems that it is hardly horrible.

The other problem here is that we seemed to have gone from looking at my ES/SDEV number and evaluating their significance/or lack of significance as it relates to case volume to what is good and bad ES. So I think we are off topic.

Kennyg – read this thread for answer to your question.
http://forum.accurateshooter.com/index.php?topic=3792461.msg36101278#msg36101278

 

[jlow]

OK, that helps. Actually I was looking more at your chrono data and was having the same thought as you. The problem I have is unless you have at least 3 chrono, it’s kind of hard to figure out if you are having problem with the chrono and which one is an outlier. Any thoughts.

 

[jlow]

To follow up on this question of the potential problem as it relates to the chronograph, there are actually two questions. One is accuracy, the other is precision. Accuracy is the ability of the chrono to tell you the true speed of the bullet. Precision is the ability of the chorno to report to you the same number for bullets of the same speed. So the two are related but NOT the same thing.

The problem here is that it is very difficult if not impossible to evaluate either one of these two without having on hand bullets that have a known MV and to have the ability to reload bullets with this MV in a reproducible fashion. What we have are bullets with unknown MV and they vary with different degree from bullet to bullet….