Tweaking bc’s

david g ranes

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On the average what have you found you have had to tweak your ballistic coefficient plus or minus to get your load velocity to match up with your come up clicks to where your impacts are I had never had any trouble in the past when using gunwerks app but since it is no longer available I have had trouble with the one I use now. The velocity I use is off a magneto speed and it has always been spot on with their app but now it shows my velocity is a lot faster with this app to make it come out been fighting it several months shot at 800 today I had to dial 1 moa less to hit from what the app says. Thanks for some input. David
 

Chase723

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Make sure your inputs for scope height, and zero offset, etc are correct and not just estimates. Use an app like Ballistics X to measure your group size and offset. You can easily be off by .2-.3 mils with those two things combined. Also make sure your DA is correct.
 
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Chase723

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Once you know all that is correct, shoot to 500 and verify your velocity. If it doesn’t match what your adjustment should be then true your velocity. Then shoot as far as you can (ideally transonic but that’s not always possible) and true your BC.
 

yobuck

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Once you know all that is correct, shoot to 500 and verify your velocity. If it doesn’t match what your adjustment should be then true your velocity. Then shoot as far as you can (ideally transonic but that’s not always possible) and true your BC.
Well the only purpose of shooting over a chronograph is to get a velocity number for creating a chart.
Its always been advisable to confirm and correct as needed by shooting.
That then becomes the correct velocity, at least for those conditions.
What else is actually required? Who cares what the B/C is or isnt so long as we have the dialing information?
And the only way to confirm anything it to shoot.
In the old days they called it a sighter shot.
 

david g ranes

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Well the only purpose of shooting over a chronograph is to get a velocity number for creating a chart.
Its always been advisable to confirm and correct as needed by shooting.
That then becomes the correct velocity, at least for those conditions.
What else is actually required? Who cares what the B/C is or isnt so long as we have the dialing information?
And the only way to confirm anything it to shoot.
In the old days they called it a sighter shot.
That’s the way I have done in the past just shoot at different yardages keep a record of my come ups but they always have been right on with gunwerks ballistic app but that isn’t holding true with geo ballistics and when I talked to John there he said to adjust the bc till it comes out but should you have to adjust by 20 or 30 that’s the question. Or is that out of line and I’m missing something David
 

dfanonymous

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That’s the way I have done in the past just shoot at different yardages keep a record of my come ups but they always have been right on with gunwerks ballistic app but that isn’t holding true with geo ballistics and when I talked to John there he said to adjust the bc till it comes out but should you have to adjust by 20 or 30 that’s the question. Or is that out of line and I’m missing something David
You don’t need to do all that. BC are models, and models aren’t direct representation of a particular bullet. So there’s some tweaking that comes into play. However, that’s at and beyond transonic.

For your purpose, the MV just needs to be tweaked for the software to line up with reality. Even if it’s a accurate and correct MV from magnetospeed.

So use the correct BC for now. Adjust MV.
 

MagnumManiac

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I use Ballistic AE in my iPhone and I point my bullets or trim, so the BC had to be calculated using drop and velocity.
The app is not always spot on, but close.
I have also found that my BC out of my 338-416 Rigby Improved has higher BC with all bullets due to it’s higher velocity.
It’s not huge, but is different.
Without altering bullet profiles, Ballistic AE is very good and accurate with the listed bullets in the app archive. The BC’s are based on Litz’s data.

Cheers.
 

david g ranes

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The the guy that made the app told me to tweak my bc’s never had to do it before just trying to figure out what’s different wish gunwerks had left their app alone I got along with it really well. David
 

david g ranes

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The only reason I’m questioning it is I don’t have access to shoot at every given range so I would like to be able to fill in the gaps with a ballistic app but what is coming up isn’t with in minute of coyote and that all I care about. David
 

MagnumManiac

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^^^^^
This is a good observation and won’t hurt anything, other than using up components. A test at 600 should give you all the data you need, if you can get out that far?

I just sighted in my new 300RUM this past weekend, took 3 shots, that leaves me with 17 to play with. Ballistic AE is telling me my MPBR is just under 400yds if I sight in at 3” high @ 100yds. I don’t think I need that much, so settled on 2.75” high @ 100.
Hope these Barnes 165g TTSX’s work.

Cheers.
 

david g ranes

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Are you positive that your scope tracks true? Might be giving you more elevation than your dialing? Time for a tall target test.
I’ve had 2 different scopes on the rifle went back to the first one I thought it might of went bad but it was a proven scope that was taken off of gun I sold and had worked fine David
 

308win

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I’ve had 2 different scopes on the rifle went back to the first one I thought it might of went bad but it was a proven scope that was taken off of gun I sold and had worked fine David
Generally, if I hit higher than calculated dope I question the scope. If lower, I question the BC. Not always the case, because scopes can also not adjust enough. Bullets tend to have inflated BC's already, so I would question adjusting BC up to match results?
 

Hand Skills

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I have found tweaking velocity at 440yd and BC at 880yd gives me the most for the least, or to say it another way, gets my solution very close with minimal inputs.

Always check the scope on a tall target first to verify tracking before moving out to distance.
 
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