Darn....lol
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Chronographs: What to do?
- Thread starter GLTaylor
- Start date
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What I do is to shoot five and take the median value as the muzzle velocity for that load. The median and the mean can be two different values. I prefer the median because I believe it represents a more accurate description of the population. A mean is too easily distorted by one or two high velocities; the median attenuates oddball velocities.
The link below is how I calculate the statistics for my loads. You just input the number for how many samples & click Submit, enter the velocities & click Calculate to see the stats in scientific notation. So easy, even a rural Nevadan can do it...
http://www.physics.csbsju.edu/stats/cstats_NROW_form.html
Yes, you always need to validate your drop at long range. There are too many variables at play, including muzzle velocity, ballistic coefficient, temperature, pressure, altitude etc. Published BCs may not be reliable, and the other factors can vary from hour to hour. You can true your trajectory by varying all these in your ballistics program, including BC.
WildRose
Well-Known Member
Unless you're setting them up wrong or plugging figures into your programs errantly my guess is that the problem is in your reloads.
If you don't have your ES and SD down into low double or even single digit range it's going to be difficult to come up with accurate calculations for your dope beyond about 600yds with most rounds.
You also have an issue that BC's are not static and may vary from load to load, rifle to rifle so you need to verify your drops at longer ranges (I like 300, 500, 700yds) and then tweak your BC in your programs to get your predicted data to match your real world measurements.
I'm giving you two thumbs-up for that one! The SD is a number that tells you how similar are the velocities in the population. If it's low (less than 30 fps out of 3000 fps; 1%), that ammo is very predictable. If it's high (three to four percent), the ammo is not very consistent and will show that on the target. I should think you'd get high SDs when the charge is both too high and too low.
Thanks for the replies fellows. I guess I should've given u a little more information.
While I'm by no means an expert, I do a great deal of shooting and totally understand and have a pretty good handle on SD's and loading practices. I have a private range and shoot 1000-1760 yds on a regular basis.
I am a fan of Berger bullets and pretty much shoot them in all my builds.
That being the case I doubt I have anything to add to the extensive BC testing that Bryan Litz has done on the bullets I'm shooting so when plugging the information in my app I use Litz' BC's.
That only leaves the velocity to tweek.
This is where I have to true my velocity as the speeds that the three devices we are discussing is never close enough to get me on target down range. Just wondered if anyone else had this issue?
Even if you used a G1 or G7 BC that has been accurately calculated to three decimal points by Litz, it still might not be good enough to get you on target at long range. The reason is because the BC is not a drag function for your projectile, it is a scaling factor applied to another "standard" projectile, which will have different ballistics to yours.
100 years or so ago, before electronic chronographs and radar, establishing a drag curve for a projectile was a slow and complicated process. To make things easier they settled on a few different standardised projectiles, established their drag curves, and for all other projectiles they multiplied these standard curves by a scaling factor called the Ballistic Coefficient.
For many years commercial bullet makers compared their projectiles to the G1 standard, and based their BCs on the G1 drag curve. The problem with this is that the G1 projectile is fairly blunt, and not a good representation of modern streamlined projectiles. To get around this some manufacturers quoted multiple BCs for different speed zones, to better match their projectile to the G1 drag function.
Litz, and maybe others, popularised the fact that the G7 standard projectile (which is a sharp, streamlined shape) was a better approximation for modern long range bullets, so he set about with his AB team accurately measuring G1 and G7 BCs for Berger and other commercial bullets.
The problem remained however, that the G7 model was still not an exact model for all the bullets he was measuring, maybe not accurate enough to get you on target at long range. He got around this problem by using radar to measure the entire drag curve of each individual bullet. If you use the AB software it includes these custom drag functions for many bullets.
However, it was then discovered that the BC of a given bullet can vary between different rifles. For example, twist rate and stability can effect the BC. Litz and AB now get around this limitation with their mobile ballistics lab by measuring your particular load in your rifle, and giving you an individualised custom drag curve. Even still, their measured BC varies slightly from shot to shot with the same bullet, in the same rifle.
So I guess the moral of this story is that there is some inevitable variation and inaccuracy in ballistic calculations. Then you have to deal with the environment....
cohunt
Well-Known Member
One nice thing about the lr is that actual bc numbers can be calculated for your rifle/bullet/weather conditions as you get multiple velocities at different distances therefore the BC can be calculated
