Truing of BC

[Tree]

As a newer rifle shooter I have been doing a lot of research about ballistics, the various apps available, capturing and recording data etc. I am trying to find info on how you true BC. I have done searches but unfortunately not having any luck. I am sure there is info on here about how to do it and if someone could point me to the info that would be greatly appreciated.

 

[molzarella]

Tagging myself in this

 

[_Windrider_]

It’s simple. If you know your MV adjust the ballistic calculator BC until the drop in the calc equals what it actually was. The published BC is only valid at the specified muzzle velocity. If you don’t have a chrono and don’t know your actual MV it’s a bit more difficult. In this case you can leave the BC and tweak your MV to get the actual drop. This is the less accurate method.

 

[Lowlight]

You want to true your BC around 800 yards, that seems to be a good middle ground.

Right now I am playing with the Utility in ColdBore to do it, and it's working out well

 

[molzarella]

You want to true your BC around 800 yards, that seems to be a good middle ground.

Right now I am playing with the Utility in ColdBore to do it, and it's working out well

View attachment 6877694

Where did you get this program?

 

[Lowlight]

This program has been around Longer than Most, it's easily the 3rd Longest running program out there

Patagonia Ballistics

Super accurate, super robust, not expensive, the only downside, is, the only App they have is for a Windows Phone. One license runs on a Windows Desktop, Windows PDA (Trimble) or Windows Phone. All linked via the One Drive

 

[molzarella]

awesome, thank you

 

[Tree]

Thanks for the info guys. I also have Coldbore on a windows phone but haven't started doing much with it other than looking around the app. I want to first get more info into my data book going before messing around too much with software. Easier for me to learn if I write it down then do the computer stuff. I do have a chrono so will be getting MV once it warms up a bit up here. Will have to make find a range to get out to 800 with as my local only goes to 400. Thankfully we do have some that go to at least 1000k within driving distance.

 

[Nik H]

You want to true your BC around 800 yards, that seems to be a good middle ground.

Right now I am playing with the Utility in ColdBore to do it, and it's working out well

View attachment 6877694

Frank,

I am a little confused.

1. You said it should be done at 800 yards but it looks like 200 yards from your entry.
2. How are you measuring velocity at 200 yards? Can't be a labradar??

 

[_Windrider_]

Frank,

I am a little confused.

1. You said it should be done at 800 yards but it looks like 200 yards from your entry.
2. How are you measuring velocity at 200 yards? Can't be a labradar??

I think those r just the defaults. It says def next to the ranges

 

[Nik H]

I think those r just the defaults. It says def next to the ranges

Got it. I never use that part of CB since I don't have anything to measure velocity at distance.

Questions still stands though. It requires a velocity at your final distance. How do you get that?

 

[Tree]

You want to true your BC around 800 yards, that seems to be a good middle ground.

Right now I am playing with the Utility in ColdBore to do it, and it's working out well

Frank, I have seen that screen in CB however it does appear to need 2 velocities to get the BC in that section. Would I do the adjustment of BC to get the true number under the ballistics section of CB since I am not currently able to get 2 velocities? Also how does DC play into truing your ballistics?

 

[Lowlight]

If the DC requires more than 10% change you adjust the input the DC is a fine tune adjustment.

I use the drop to get the velocity and you can check it with the TOF option. I did not get a second velocity I used it out of the range card based on elevation

I adjusted it down using the programs adjusted number.

 

[Nik H]

If the DC requires more than 10% change you adjust the input the DC is a fine tune adjustment.

I use the drop to get the velocity and you can check it with the TOF option. I did not get a second velocity I used it out of the range card based on elevation

I adjusted it down using the programs adjusted number.

Brilliant...thank you

 

[Tree]

Ok so I am probably overthinking this or doing something wrong but I went into CB to test this out.

Using the Prime Ammo info for a 130gr 6,5 Creed I input the parameters into the ballistics section to get the drop data.

I then adjusted the BC down on the original input data to simulate a different drop at 800 yards.

Using the ending velocity number at 800 I put it into the BC from velocity in analyzer, I also put the TOF from the 800 yard line and input it into BC from TOF. Doing both of these gave me 2 separate G1 BCs. BC from Velocity - .458 BC from TOF - .476. Any insight would be greatly appreciated.

 

[Nik H]

I noticed you are using 2751 fps for MV. Where did that come from?

 

[Diver160651]

You want to true your BC around 800 yards, that seems to be a good middle ground.

Right now I am playing with the Utility in ColdBore to do it, and it's working out well

View attachment 6877694

3 things to add...

1. Before doing anything, please please make 100% certain your range is correct. Do not assume the range figured it out/
2. Make darn sure "you" are not tuning in a new shooter offset.
3. Make sure your short range 300/400 velocity is correct.

 

[tna9001]

I haven't used Applied Ballistics through the transonic regime but inside of it (around 1300 yards for my 6.5CM) the custom drag curves don't need to be trued, they are spot on. I've measured vertical dispersion at 1,000 yards using AB's custom curves and got right around 6" over 3 different outings.

 

[Tree]

I noticed you are using 2751 fps for MV. Where did that come from?

That would be an ID10T error. See I knew I was doing something wrong.

 

[Tree]

I noticed you are using 2751 fps for MV. Where did that come from?

After correcting I still received different BCs from the BC from Velocity and BC from TOF. .462 and .479 respectively.

3. Make sure your short range 300/400 velocity is correct.

Can you clarify this for me? What do you mean make sure short range velocity is correct?

 

[Nik H]

After correcting I still received different BCs from the BC from Velocity and BC from TOF. .462 and .479 respectively.

There is probably a good explanation for this but it is beyond me. I honestly don't obsess much about this stuff anymore. I use CB to get me close and record DOPE.

 

[Diver160651]

Can you clarify this for me? What do you mean make sure short range velocity is correct?

Let’s assume you can shoot a group just over an inch at 400.

Assuming the target is “Really” at 400.. if your group is not dead center your velocity is not correct. BC has very little effect compared to gravity at this distance.

FFS and a few others allows you to measure the exact offset and recalculate the velocity. Then further down the road you tune BC. The DK, is much further down the road on the ELR side

 

[Tree]

Thank you for that explanation, That is how I interpreted lowlights post about getting velocity from drop to mean so just wanted to ensure there wasn't something else I might be missing.

 

[Christopher Rance]

Let’s assume you can shoot a group just over an inch at 400.

Assuming the target is “Really” at 400.. if your group is not dead center your velocity is not correct. BC has very little effect compared to gravity at this distance.

FFS and a few others allows you to measure the exact offset and recalculate the velocity. Then further down the road you tune BC. The DK, is much further down the road on the ELR side

For teaching purposes; the steps that I should follow are;

1. Mount scope. Level off plumb line, check cant.
2. Conduct scope calibration @ 100 meters. Note any correction factor on turret dials.
3. Group and zero. Conduct true zero range input if needed. Use chronograph to measure MV and note SD.
4. Set up a target, 400 meters away, confirm distance, shoot a five round group, annotate exact POI, measure exact offset and recalculate MV if needed.
5. Shoot out to distance; I assume out to transonic, fine tune BC if actual drop is not matching predicted drop.
6. If shooting into subsonic, tune DK if actual/predicted drop is off.

As always, use a data book. Record shots, environment, call your shots ect..

 

[Nik H]

Let’s assume you can shoot a group just over an inch at 400.

Assuming the target is “Really” at 400.. if your group is not dead center your velocity is not correct. BC has very little effect compared to gravity at this distance.

FFS and a few others allows you to measure the exact offset and recalculate the velocity. Then further down the road you tune BC. The DK, is much further down the road on the ELR side

I am assuming that there is nothing magic about 400 yards/meters. You could do the same thing at 500 yards...correct?

 

[Diver160651]

I am assuming that there is nothing magic about 400 yards/meters. You could do the same thing at 500 yards...correct?

Yes, actually for the faster higher BC bullets, that is actually a bit better. All we're after is far enough to accurately measure the drop, before the BC has a large impact. With optical and acoustic, this was absolute necessity, sometimes with even the labradar or Magneto speed, the program still may want a velocity change. By making sure this is correct, when you work the BC you'll have a handle on both parts of the curve.

That's were the log book with a call column and an actual column is so helpful when attempting to curve fit.. Because after you have all the stepped out and confirmed drops you can see how your changes effect the curves at both ends when back on your desktop.

BTW here is an example of my log book: See the various ranges with columns for the Solvers on the right half "Call" and the "ACT" actual centered up impact.

 

[Tree]

Thank you Diver160651, C. Rance, and lowlight for the information. What I was hoping to get out of the discussion (which I did) was to learn what I needed to do at the range and what data to gather before I enter anything into any software. I can't get out to the range as often as I would like so making the most of the time I do get out will help.

 

[Nik H]

Yes, actually for the faster higher BC bullets, that is actually a bit better. All we're after is far enough to accurately measure the drop, before the BC has a large impact. With optical and acoustic, this was absolute necessity, sometimes with even the labradar or Magneto speed, the program still may want a velocity change. By making sure this is correct, when you work the BC you'll have a handle on both parts of the curve.

That's were the log book with a call column and an actual column is so helpful when attempting to curve fit.. Because after you have all the stepped out and confirmed drops you can see how your changes effect the curves at both ends when back on your desktop.

BTW here is an example of my log book: See the various ranges with columns for the Solvers on the right half "Call" and the "ACT" actual centered up impact.

Much obliged for all of the help Diver

 

[Possumpete]

Good thread going here guys,lots of good info thanks.

 

[Donelan47]

Tag for future reference

 

[seansmd]

This is the exact process @Diver160651 walked me through live at the range. I was using the kestrel 5700AB with the custom curve so didn't have the ability to tune the BC, just the MV. We trued the MV at 575 yards, went out to 1000 using the adjusted MV in the kestrel and the drop was right on. Once this was complete the drops were perfect from 100 out to 1000, with many odd steps in between at his steel range.

 

[ShootnFool8]

Great info guys! Wandered myself on where to start, or how it's done.

 

[DocUSMCRetired]

Here is a video that explains how to properly do a ballistic calibration: