BC Advice - Published vs Calculated vs Custom Drag Curve ???

[rustyinbend]

What's the conventional wisdom out there regarding the BC to plug into a Kestrel? The choices I'm looking at are:

1. The number on the bullet box published by the manufacturer
2. The number calculated by the JBM Ballistics Calculator using actual down-range velocity from my Labradar
3. The number that Kestrel recommends when selecting their Custom Drag Curve during setup

For my MPA 300 Win Mag Berger Hybrid Target 200.20x projectiles, I get:

1. Published: .328 (G7) ... on the box
2. Calculated: .315 (G7) ... using my measured velocity and the JBM Calculator
3. Custom Drag Curve: It's a secret - the Kestrel says "1.0" on all Custom Drag Curves

I know ... I know ... the standard advice will be "Shoot it and see which one gives you on-target tight-groups".

I'm just interested to know what the heavy-hitters on this forum tend to trust more, and use regularly. Which of these have you guys found to be most accurate?

 

[XLR308]

I use published innitially just as a standard to start with then adjust based on impact.
Unless im using some of Noslers stated BC numbers i just ignore them and start from scratch

 

[6.5SH]

What's interesting to me is your JBM/LabRadar result being lower than what was published. Usually because of the short distances involved it will be higher.

 

[Huskydriver]

Once I trye mv all the kestrel custom curves have been dead nuts out to 1200 for me across 4 calibers and 6-8 bullet types

 

[OREGUN]

Until you have verified by shooting out past 600 yards or so, whatever you put in the Kestrel is just a guess. Use the published or your LabRadar number, or any other number you find. Once you have verified, adjust the Kestrel to match reality as necessary.

 

[OREGUN]

Advanced Marksmanship - Going Inside the Numbers

You’ve got your new precision rifle and you’ve downloaded the latest – greatest ballistic solver. You are coffee’d up, it’s a beautiful day and you are off to the range to gather data. But really. Really. Do you understand what you are doing? Most do not. Many shooters attempt to create...

www.snipershide.com

Read this. Especially the last section.

 

[Rocketmandb]

I know ... I know ... the standard advice will be "Shoot it and see which one gives you on-target tight-groups".

This is really the best thing to do - except forget the tight group thing, just focus on what gets you closest to center. Besides, I find some of the most fun shooting to be when I don't use any equipment to set the shot. Set up a target, calculate the distance using my reticle, go to my DOPE and take a shot. Very rewarding.

And DOPE doesn't lie.

 

[badassgunworks]

None of them truly work the way they're supposed to you basically need three different profiles for three different range distances. Super Sonic, transonic, subsonic

 

[jasent]

Like everyone else says shooting it and finding out how your rifle affects the bc is the correct way. I’ve had good results with custom drag curves but they are not perfect and I have had to true it up. Like mark said shoot at a super sonic range, then subsonic and true from there

 

[Steel head]

Yea
You just gotta see what works for you.
Get some good dope and sort it out, it’s part of the game.

CDM’s seem to work well with Berger’s but you’ll still have to true up a bit.

 

[OREGUN]

With all that said, I just spent a weekend getting rock solid dope out to 1450 yds.

When I went to true up the Kestral (AB), the muzzle velocity and BC (g7) was within 6 FPS of my magnetospeed and .004 of the box value BC So, those are probably good places to start.

 

[rustyinbend]

Advanced Marksmanship - Going Inside the Numbers

You’ve got your new precision rifle and you’ve downloaded the latest – greatest ballistic solver. You are coffee’d up, it’s a beautiful day and you are off to the range to gather data. But really. Really. Do you understand what you are doing? Most do not. Many shooters attempt to create...

www.snipershide.com

Read this. Especially the last section.

Excellent article ... thanks for the recommendation.

 

[rustyinbend]

With all that said, I just spent a weekend getting rock solid dope out to 1450 yds.

When I went to true up the Kestral (AB), the muzzle velocity and BC (g7) was within 6 FPS of my magnetospeed and .004 of the box value BC So, those are probably good places to start.

Makes sense ... thanks.

So far I have (a) figure it out on your own, (b) custom curves are spot-on, and (c) the BC on the box is righteous.

I'm heading to a long-range class in Utah this week, with a load I've got totally dialed in at 100-yards with SD's of 6.5 and ES's of sub-20 ... and the groups are great. My problem is that I haven't been able to test and calibrate out past that ... other than to hit an 18" piece of steel at 500 yards with all three (published BC, custom curve, calibrated BC).

• MPA 300-WMBA
• Peterson 1x brass
• Federal premium match LR Magnum primers
• Accurate MagPro (78.4 gr)
• Berger 200.20x Hybrid Target

So I'm trying to figure which of these ballistics models to start with when I get to the firing line. I have all three programmed into my Kestrel. Here's the logic I'll probably use (assuming I can do all this within the framework of the class):

1. Start with the published BC (.328)
2. If that works ... stay there.
3. If hitting high at distance, switch to the calculated BC (.315) and see if it brings me down to the targets
4. If that works ... stay there.
5. If low or high with calculated BC ... switch to Custom Drag Curve
6. If that works ... stay there.
7. If it doesn't work ... stick with the CDC and adjust DOPE to get on target
8. If none of that works ... melt down my rifle and make golf clubs.

Been looking forward to this class since I scheduled it pre-pandemic ... and had to push it out over a year. Originally scheduled in June of 2020, but that didn't happen for obvious reasons.

 

[lte82]

Makes sense ... thanks.

So far I have (a) figure it out on your own, (b) custom curves are spot-on, and (c) the BC on the box is righteous.

I'm heading to a long-range class in Utah this week, with a load I've got totally dialed in at 100-yards with SD's of 6.5 and ES's of sub-20 ... and the groups are great. My problem is that I haven't been able to test and calibrate out past that ... other than to hit an 18" piece of steel at 500 yards with all three (published BC, custom curve, calibrated BC).

• MPA 300-WMBA
• Peterson 1x brass
• Federal premium match LR Magnum primers
• Accurate MagPro (78.4 gr)
• Berger 200.20x Hybrid Target

So I'm trying to figure which of these ballistics models to start with when I get to the firing line. I have all three programmed into my Kestrel. Here's the logic I'll probably use (assuming I can do all this within the framework of the class):

1. Start with the published BC (.328)
2. If that works ... stay there.
3. If hitting high at distance, switch to the calculated BC (.315) and see if it brings me down to the targets
4. If that works ... stay there.
5. If low or high with calculated BC ... switch to Custom Drag Curve
6. If that works ... stay there.
7. If it doesn't work ... stick with the CDC and adjust DOPE to get on target
8. If none of that works ... melt down my rifle and make golf clubs.

Been looking forward to this class since I scheduled it pre-pandemic ... and had to push it out over a year. Originally scheduled in June of 2020, but that didn't happen for obvious reasons.

Except your number 3 is wrong. If he uses .328 and hits high he would want to increase the BC in the kestrel, not decrease it.

 

[rustyinbend]

Except your number 3 is wrong. If he uses .328 and hits high he would want to increase the BC in the kestrel, not decrease it.

Wait, what? Here's what I thought ... the higher the BC, the lower the drag, the higher the impact.

Right?

So a BC of .328 should hit higher at distance than a BC of .315 ... meaning if I'm hitting high with a .328 BC, calculating for a .315 BC should lower my impact on the target.

What am I missing?

 

[Steel head]

Wait, what? Here's what I thought ... the higher the BC, the lower the drag, the higher the impact.

Right?

So a BC of .328 should hit higher at distance than a BC of .315 ... meaning if I'm hitting high with a .328 BC, calculating for a .315 BC should lower my impact on the target.

What am I missing?

Your #3 statement reads opposite
Yes for a certain speed if say your running a .300 G7 and you hit high consistently you could look into a higher BC number.

 

[rustyinbend]

Your #3 statement reads opposite
Yes for a certain speed if say your running a .300 G7 and you hit high consistently you could look into a higher BC number.

Sorry guys ... I'm really confused.

The higher the BC, the lower the drag, the less the drop, the higher the impact.

Is that statement wrong?

If that statement is correct, then it stands to reason the opposite.

The lower the BC, the higher the drag, the more the drop, the lower the impact.

Have I learned this all incorrectly?

 

[OREGUN]

If it’s a class…let them teach you. It’s really unlikely that any published number is going to match reality….you will have to “true” your chosen ballistic calculator no matter which number you use. Published BC numbers are based on velocity and take time to become reality (why it appeared to not matter at 500 yds…not enough time had elapsed) so unless your rifle shoots exactly like the test gun and you are in the exact same atmospheric conditions, the numbers won’t be exact for you.

If it was me, I would use the CDC and then do what your instructors recommend to verify your dope and then, finally true the kestrel.

 

[lte82]

Sorry guys ... I'm really confused.

The higher the BC, the lower the drag, the less the drop, the higher the impact.

Is that statement wrong?

If that statement is correct, then it stands to reason the opposite.

The lower the BC, the higher the drag, the more the drop, the lower the impact.

Have I learned this all incorrectly?

Put a .328 into the kestrel and say it gives you a solution 8 mil at 1000, but you are impacting .2 mil high. Changing your BC to .315 and it will tell you your solution should be 8.1or 8.2, making your impacts even higher. To make the correct correction, you’ll need to increase the BC in the kestrel so it spits out a solution that matches what you really need.

 

[Steel head]

If it’s a class…let them teach you. It’s really unlikely that any published number is going to match reality….you will have to “true” your chosen ballistic calculator no matter which number you use. Published BC numbers are based on velocity and take time to become reality (why it appeared to not matter at 500 yds…not enough time had elapsed) so unless your rifle shoots exactly like the test gun and you are in the exact same atmospheric conditions, the numbers won’t be exact for you.

If it was me, I would use the CDC and then do what your instructors recommend to verify your dope and then, finally true the kestrel.

Barrels, brakes and silencers affect it as well so every rifle has its own perfect curve.
Out to roughly transition Berger has been quite close for me lately.

That would be cool to study effects on BC from individual bits of the rifle.

 

[rustyinbend]

Put a .328 into the kestrel and say it gives you a solution 8 mil at 1000, but you are impacting .2 mil high. Changing your BC to .315 and it will tell you your solution should be 8.1or 8.2, making your impacts even higher. To make the correct correction, you’ll need to increase the BC in the kestrel so it spits out a solution that matches what you really need.

I had to read that about 8 times for my peanut brain to digest it completely.

So if that's true, and I can't dispute that it is ... if I'm hitting low with a .328 BC, switching to the .318 BC should give me elevation that will actually raise my point of impact.

This is exactly "opposite" of what I was thinking ... thanks for the calibration (no pun intended). I think I get it now. I wasn't thinking about the Kestrel solution as a function of BC ... I was mindlessly thinking about the raw ballistics.

 

[Steel head]

Put a .328 into the kestrel and say it gives you a solution 8 mil at 1000, but you are impacting .2 mil high. Changing your BC to .315 and it will tell you your solution should be 8.1or 8.2, making your impacts even higher. To make the correct correction, you’ll need to increase the BC in the kestrel so it spits out a solution that matches what you really need.

That’s actually wrong

If your solver is telling you you need 8 mils but you hit .2 high then you really needed to dial 7.8 and to get that you’d increase speed or BC

 

[Steel head]

Sorry guys ... I'm really confused.

The higher the BC, the lower the drag, the less the drop, the higher the impact.

Is that statement wrong?

If that statement is correct, then it stands to reason the opposite.

The lower the BC, the higher the drag, the more the drop, the lower the impact.

Have I learned this all incorrectly?

You are correct

how you worded it originally in #3 was just off.

 

[lte82]

That’s actually wrong

If your solver is telling you you need 8 mils but you hit .2 high then you really needed to dial 7.8 and to get that you’d increase speed or BC

Lol. Go re read it. That’s precisely what I wrote.

 

[Steel head]

Lol. Go re read it. That’s precisely what I wrote.

Doesn’t like like it to me

maybe you should go try that on your kestrel.

 

[b6graham]

Doesn’t like like it to me

maybe you should go try that on your kestrel.

I reread what he said. Goofy wording aside he was correct. Saying switching to the .318 BC dialing 8.1-8.2 will make the impact even higher on target (because you dialed extra .1 or .2 for something that was already high)

 

[Steel head]

I reread what he said. Goofy wording aside he was correct. Saying switching to the .318 BC dialing 8.1-8.2 will make the impact even higher on target (because you dialed extra .1 or .2 for something that was already high)

But that would raise the impact even more.
call for 8, hit .2 high so you really needed 7.8

if it hit .2 low then lowering the BC would be called for and you then would dial
8.2.

Put a .328 into the kestrel and say it gives you a solution 8 mil at 1000, but you are impacting .2 mil high.

 

[lte82]

But that would raise the impact even more.
call for 8, hit .2 high so you really needed 7.8

if it hit .2 low then lowering the BC would be called for and you then would dial
8.2.

Yes. Exactly.

 

[b6graham]

Yall just wording shit funny. But you're both saying the same thing

 

[Steel head]

I can understand what @lte82 is saying till he said impact.
At the that point your now working with a result and not a computation, then you adjust to the result not keep fiddling with unverified computations

 

[lte82]

I can understand what @lte82 is saying till he said impact.
At the that point your now working with a result and not a computation, then you adjust to the result not keep fiddling with unverified computations

If you impact high, you have to increase your BC or velocity in the kestrel. If you impact low, you have to decrease your BC or velocity in the kestrel. All of that assuming your starting zero is correct. It's all very simple...

 

[OREGUN]

Gawd this is painful.

 

[rustyinbend]

Gawd this is painful.

Hey ... don't blame me. I long ago stated that I understood the correct answer. LOL

 

[The4GunGuy]

What's the conventional wisdom out there regarding the BC to plug into a Kestrel? The choices I'm looking at are:

1. The number on the bullet box published by the manufacturer
2. The number calculated by the JBM Ballistics Calculator using actual down-range velocity from my Labradar
3. The number that Kestrel recommends when selecting their Custom Drag Curve during setup

For my MPA 300 Win Mag Berger Hybrid Target 200.20x projectiles, I get:

1. Published: .328 (G7) ... on the box
2. Calculated: .315 (G7) ... using my measured velocity and the JBM Calculator
3. Custom Drag Curve: It's a secret - the Kestrel says "1.0" on all Custom Drag Curves

I know ... I know ... the standard advice will be "Shoot it and see which one gives you on-target tight-groups".

I'm just interested to know what the heavy-hitters on this forum tend to trust more, and use regularly. Which of these have you guys found to be most accurate?

I'm no "heavy hitter", but I can tell you that for BOTH my 6.5 Creedmoor using 140gr Hornady ELD-M's and my Vudoo V22 22LR (using Lapua Center-X and SK Match) I have totally gone to the custom curve. I was shocked when I entered my velocity for the 6.5 CM and was immediately vertically centered at the transonic range.

 

[Im2bent]

Its simple lets say you have bullet x and the box says bc is .500. You send it and its high. This means your bullet has less drag than expected or a HIGHER bc. You put a higher bc in the kestrel it says oh you have a faster bullet so you need less up adjustment.

 

[Sm1990]

Ab custom curve has been spot on with the 105 hybrids for me.

 

[Graham s]

I started with published and i still use it till date .