Need quick answers to Semi-noob questions

[300ATT]

Background:

I have an AI/AT in .243 that shoots decent (1/4"-3/8"). I also have good muzzle velocities and a good 100 yard zero. I am shooting 105 Berger's and used Shooter App on the iPhone to predict drops and then went out and measured the actual drops from 300 - 1,328 yards on steel. I found that I needed 0.2 - 0.3 mil LESS elevation than Shooter predicted.

Questions:

1.) What is the best BC to use: g1, g7 or Brain Litz's g7 numbers?

2.) What's the best way to "curve fit" the output from Shooter to match the observed results?
a.) Muck with the BC at various velocities?
- or -
b.) inflate the velocity?
- or -
c.) A little of both? If so how much to play with velocity vs. BC to get a matchup?

Thanks!!

 

[spife7980]

If you have a known velocity from a quality chrono then I would adjust the BC. Whats the point of getting a good chrono number if you change it anyways?
I dont use shooter but in ballisticsarc I can use an preconfigured litz bc number or a manufacuter bc number or I can enter my own details. I enter my own details as thats how I can edit the bc. Im using g7 ICAO. Took my 105 rdf from a .280 to a .310 to line up at 1k.

 

[300ATT]

Thanks spife for answering:

I tend to agree. have a good chrono and hate to change observed data - like saying the group was really only 4" at 400 when it was 8". I will play with banded BC's to see where that gets me. I heard Frank G. mention somewhere that it is easier to line up observed with data coming from the software if you go with a G1 bc. What have others experienced?

 

[spamassassin]

Start by being more precise in your query. Berger makes at least 4 different 6mm 105's.

Make sure your inputs are exactly right and that you're using the BC for the exact bullet you're using, verify your MV's on each shot you take to gather drops since you're going to base calculation tweaks on the information gathered, double check calibration of atmosphere measuring tools, make sure there's no shooter error inducing consistent high hits.

G1 vs. G7, when used intelligently I've not seen any useful difference. The calculator that I publish uses G1 BC's and always returns good data with good inputs. What has to be dealt with is the fact that BC is only really accurate for a particular window of mach values. The ballistic calculators I make (shameless plug, ballisticxlr.com) uses BC banding so you don't have growing inaccuracy of calculation as you depart from the spec window of mach that the manufacturer of the bullet had in mind when they chose what BC to publish.

I don't advocate messing with base input values but that's how many makers of ballistic calculators have chosen to deal with this issue. That's just going to pollute results somewhere else in the resulting data set. My route was to enable the calculator that I make to deal with the fact that the BC number is related to mach number. It was a HUGE change to how the thing worked but was worth it.

 

[Mordamer]

I have had excellent results using Strelok by plugging in the average velocity from my magneto speed and the truing the G1 BC at long distance. I did this at 1300 yards with my match rifle after I developed my load. I have gone to 3 matches with it since and I have never had to touch anything besides atmospheric data in the calculator and I have been spot on for elevation from 300 to 1,350 yards.

 

[300ATT]

Thanks for the replies and help! I will experiment with Banded BC's at different velocities.

 

[300ATT]

Sorry forgot to add - Am shooting 6mm Berger 105gr Hybrid Target bullets.

 

[DocUSMCRetired]

See if any of this helps:

1) http://www.appliedballisticsllc.com/Articles/ABDOC2.2 - A Better Ballistic Coefficient.pdf

2) http://www.appliedballisticsllc.com/Articles/ABDOC2.3 - Form Factors A Usefull Tool.pdf

3) http://www.appliedballisticsllc.com/Articles/ABDOC130_CDM.pdf

4) http://www.appliedballisticsllc.com/Articles/BallisticCalibration.pdf

5) http://www.appliedballisticsllc.com/Articles/ABDOC106_BCTesting.pdf

6) http://www.appliedballisticsllc.com/Articles/ABDOC2.1 - OGIVECOMPARISON.pdf

Also the chronograph you use can make a big difference. This is an old article we have, but it is still relevant. We recommend the Lab Radar as being the most accurate we have ever tested. http://www.appliedballisticsllc.com/Articles/ChronographChapter.pdf

You can find most of our articles here: http://appliedballisticsllc.com/ballistics-educational-resources/articles/ But I would recommend using a CDM and getting rid of the use of a BC all together.

 

[damoncali]

The simple answer is:

1) Use G7.

2) Tweak the thing with the most uncertainty. Don't mess with BC until you've measured everything else.

 

[300ATT]

Thanks for the help folks! I appreciate the links above and will study.

Am using a (old School) Oehler 35P. It would be interesting to compare its output to the Lab Radar. Not sure how much velocity you use with the Oehler 8' in front of the muzzle vs. the lab radar.

 

[DocUSMCRetired]

Thanks for the help folks! I appreciate the links above and will study.

Am using a (old School) Oehler 35P. It would be interesting to compare its output to the Lab Radar. Not sure how much velocity you use with the Oehler 8' in front of the muzzle vs. the lab radar.

Depends on the Drag of the bullet.

 

[300ATT]

Doc,

I appreciate the links above. I was reading them last night and am impressed with how well written they are. Great information! As to the chrono 8' in front of the muzzle ... everything starts slowing down the minute it leaves the muzzle (sometimes before :0 ). I believe if that slowdown for a reasonably slick bullet (105 Berger Hybrid @ 3,130 fps) "is in the noise" and would not show up as a even a rounding error in the ballistic solvers - Is this correct?

 

[DocUSMCRetired]

Doc,

I appreciate the links above. I was reading them last night and am impressed with how well written they are. Great information! As to the chrono 8' in front of the muzzle ... everything starts slowing down the minute it leaves the muzzle (sometimes before :0 ). I believe if that slowdown for a reasonably slick bullet (105 Berger Hybrid @ 3,130 fps) "is in the noise" and would not show up as a even a rounding error in the ballistic solvers - Is this correct?

It actually would show up at 8'.

 

[300ATT]

That settles it. I need a lab radar ...

 

[spife7980]

It would just be something like 12 fps faster at the muzzle. Your oehler is just fine
http://bulletin.accurateshooter.com/2018/06/chrono-camparison-test-labradar-magnetospeed-oehler-35p/

 

[DocUSMCRetired]

No chronograph on the market reads the velocity at the actual muzzle. Not even a Magnetospeed. You can also always just do the math: http://www.appliedballisticsllc.com/Articles/ABDOC121_VelocityDecay.pdf

 

[300ATT]

Great article spife.

Thanks!!