Did I do this right

[Wellington]

Looking to shoot long range with my 6br. I used this program and a chrony for reference. I measured "my" drop from 200yds to 300yds and it is not the same as the program. So... I am assuming the chrony is out. According to my readings and the program I should have had a 7.5" drop. After shooting two 5 shot groups (200 and 300) my drop is only 6.25". I adjusted the velocity in the program until I got the 6.25 drop. Is this the way you would do it? I cross checked this program with the Berger one and the results are almost identical.

ShootersCalculator.com | Ballistic Trajectory Calculator

Calculates the ballistic trajectory of a bullet fired from a rifle, handgun or other firearm. Produces a ballistic trajectory chart and table that shows the drop, velocity, kinetic energy, windage, and trajectory of a bullet.

shooterscalculator.com

 

[Bill K]

First off, I find a load or two that are the most accurate for me and my rifle.
Then I work out the drop for it, at the given ranges. They probably will be close to what your calculator shows, but may be a little different, as you found out.
Just go with what he have and use the drops figures you find or come up with to make solid hits on your target/game at the ranges you will shoot at.

 

[whatsupdoc]

No not really, you changed one number in the equation to match the results you are getting
at one particular distance.

You need to test your load at multiple distances, then you can tweak your ballistic app to match
your rifle/load combination.

Depending on the app you are using you may have to tweak the BC and velocity to get close to the actual
drop. The Hornady app for instance has a setting called axial form factor to make adjustments.

But before any of that double check that all the data you entered into the app is correct, that is very important.

 

[Fast14riot]

Also, make sure your scope height you entered into the calculator is accurate. This will affect your calculated drop.

 

[FeMan]

That is too close to get any meaningful results. The difference (0.75") is only 0.25 moa at 300 yards. Your zero, scope height, or just group dispersion could result in that much change easily with the correct inputs.

If you have a place to check it, you will get more reliable results truing your velocity inputs based on drops at 600 yards. Then do the same at 1k to trim your bullets bc input.

 

[rwj]

If you have a place to check it, you will get more reliable results truing your velocity inputs based on drops at 600 yards. Then do the same at 1k to trim your bullets bc input.

Is there enough variation in the bullet nose and/or meplat profiles to account for the bullet BC change that sometimes has to be made in ballistic calculators to match the drop actually observed?

 

[FeMan]

Is there enough variation in the bullet nose and/or meplat profiles to account for the bullet BC change that sometimes has to be made in ballistic calculators to match the drop actually observed?

I am not sure what to attribute it to, but I do see variations in BC even with the same bullet maker between lots. In my limited testing of bullets I shoot, Berger tends to be fairly consistent in trued bc between lots. I also shoot Hornady and DTACs (Sierra) and they seem to vary more. I am sure the bullet shape between lots is probably the biggest factor.

From what I have read about the study of personal drag models (PDMs), in addition to twist rate and velocity your BC is also dependent on such nuances as the powder, barrel, and other seemingly unrelated factors wich also affect the actual bc. I was fortunate enough to have Brian Litz and his crew do PDMs for two rifles. They had me do 20 shots and measured the velocity every foot out to 4,000 yards. They calculated the BC on my 184s at exactly what Berger listed and the BC on my 156s at slightly less than Berger lists (0.345 measured vs 0.347 advertised).