Do you adjust MV or BC value of a ballistic app when field proven the ballistics?

[manitou1]

That does make sense and will go that route. How come 600 is the number for changing the BC value?


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BC has a greater affect as the bullet slows down and the time in flight slows down, ie. It has more time to affect the bullet flight in the latter half of the trajectory vs the first half, especially at longer ranges. At least that is what the "experts" say.

I am sure there are other variables but it has worked for me.

We all have to settle on a "system" that satisfies us or we can go down one rabbit hole after another and we as men tend to over-analyze many things, lol.

At least it is a confidence builder.

 

[Formidilosus]

So you run straight BC values from the box and just change MV until it lines up?

I use known, measured BC values- they’re available for every good bullet. Zero correctly, then shoot at distance near transonic or as far as I will shoot with a specific rifle/bullet ever, with enough shots to see the true center of the cone, then adjust MV to match.

ETA: how much are you changing MV from chronograph results to get dope to align? 10, 20, 50, up to 100fps?

I haven’t used a chrono for a long range hunting rifle in a very long time until the Garmin. At the S2H April class I chronoed three rifles with multiple ammo types each that I had data for by trueing at distance as above, and the largest MV discrepancy was 11fps average IIRC.


It’s not that legit chrono numbers are bad, nor is it that he BC numbers are off when using known real values- it’s that people have something wrong with the system. Zero errors, MV variation, not shooting enough shots at distance to see the true average center of the cone, slight winds when shooting at distance, their scope not adjusting perfectly 100% across the entire adjustment range, etc.

Trying to adjust BC of known, Doppler radar measured bullets by shooting them, especially above terminal velocity range- is near inanity the moment you look at and understand the possible variation in BC rifle to rifle, and the tiny differences that result in target down range.

And who said anything about truing up an inch at 800 yards?

An inch at 800 yards is what results from a 10 point BC change at .536G1 and 2,940fps MV. Thats about the max that bullet BC varies from rifle to rifle unless something is very wrong.

 

[XLR]

I true BC at 1000 and then verify at 5-600 and this has never let me down. The muzzle velocity your chronograph gives you is accurate (as long as you use a good one) so the only other factor to change is BC. Your BC will change depending on what twist your barrel is and what speed you are running. Hornady publishes different Mach numbers and how the BC changes depending on your speed and twist rate. CDMs can solve some of this but I have found that running a G7 BC and adjusting it to match your data has been more accurate for me.

7mm 180 gr. ELD Match
1 in 8.75" Twist
Mach 2.25
0.391 G7
Mach 2
0.376 G7
Mach 1.75
0.368 G7

7mm 180 gr. ELD Match
1 in 7.5" Twist
Mach 2.25
0.411 G7
Mach 2
0.408 G7
Mach 1.75
0.394 G7

 

[parshal]

I use measured velocity and shoot to 600 which is the longest distance I have regular access to. I then adjust BC to match if not running a CC. Haven’t had an issue where this doesn’t work out to 1000 for Hunter matches.

Most times I just use a CC, plug in measure velocity and drops match as far as I can shoot.

 

[Formidilosus]

I use measured velocity and shoot to 600 which is the longest distance I have regular access to. I then adjust BC to match if not running a CC. Haven’t had an issue where this doesn’t work out to 1000 for Hunter matches.

Most times I just use a CC, plug in measure velocity and drops match as far as I can shoot.

How large are the errors in BC that you are adjusting out at 600 yards, and what bullets?

 

[parshal]

15-20 points at the most. Berger LRHT or Hornady ELD. Most times I run a CC though.

Had a buddy run a 6 Creed in NRL Hunter skills with the 105 (he claims). We stopped at the range on the way to the match. We had a BC of .946 to get it to match drops at 600. Dope was solid to 1000 at the match. Obviously something in the AB setup was wrong but we couldn’t figure it out and neither could some of the other folks at the match.

 

[philcox]

... If you are shooting a Hammer, then yeah- the BC is wrong.

@Formidilosus do you know if there is a good source to find BC for Hammers? I am looking specifically at the .264 124g Hammer Hunter

 

[parshal]

@Formidilosus do you know if there is a good source to find BC for Hammers? I am looking specifically at the .264 124g Hammer Hunter

I used .199 for the 124 HH. It matched from 100-600.

 

[philcox]

I used .199 for the 124 HH. It matched from 100-600.

G7 I assume?

 

[parshal]

Yes.

 

[philcox]

Yes.

Well that is a CRAPPY bullet. Based on my MV of 2900, that is 500y load in my 6.5CM. I am gonna have to get those DRT's to work it appears.

 

[ID_Matt]

7mm 180 gr. ELD Match
1 in 8.75" Twist
Mach 2.25
0.391 G7
Mach 2
0.376 G7
Mach 1.75
0.368 G7

7mm 180 gr. ELD Match
1 in 7.5" Twist
Mach 2.25
0.411 G7
Mach 2
0.408 G7
Mach 1.75
0.394 G7

This is what I came to ask about... What does a guy do in this situation? I know this is an extreme example, but you have a spread of .368 - .411 from one end to the other, so quite a bit in between to choose depending on your twist rate and MV.

 

[huntnful]

@Formidilosus do you know if there is a good source to find BC for Hammers? I am looking specifically at the .264 124g Hammer Hunter

I had a .2 G7 on my gun. Almost every one of their bullets I shot (and I shot quite a few) were .030 lower than advertised, if not more.



If you have a good chrono, and shooting a good bullet with good rifle data in regards to zero ect. the truing at 600 should damn near be non existent. I don't think I've ever had a legit zero, scope, and bullet, paired with good chrono velocity and legit environmental calculating ballistic rangefinder even really be truable at 600 yards. They hit the middle unless something in the system is off. If I was more than 3" off at 600 yards I would be looking at other things, way before I was trying to true my data.

If you don't have a chrono, and are using a good bullet, true MV. No question about it.

 

[philcox]

I had a .2 G7 on my gun. Almost every one of their bullets I shot (and I shot quite a few) were .030 lower than advertised, if not more.

This is consistent with parshal, likely legit

If you have a good chrono, and shooting a good bullet with good rifle data in regards to zero ect. the truing at 600 should damn near be non existent. I don't think I've ever had a legit zero, scope, and bullet, paired with good chrono velocity and legit environmental calculating ballistic rangefinder even really be truable at 600 yards. They hit the middle unless something in the system is off. If I was more than 3" off at 600 yards I would be looking at other things, way before I was trying to true my data.

I have a Garmin Xero. I'll use .2 and G7 in my Revic app, and see how things go.

 

[Formidilosus]

15-20 points at the most. Berger LRHT or Hornady ELD. Most times I run a CC though.

What difference in inches at what range? 20 points is ridiculously high and there is no way you are seeing a 20 point difference in actual BC if using Hornady’s BC for the bullets, and Berger’s for theirs.

Had a buddy run a 6 Creed in NRL Hunter skills with the 105 (he claims). We stopped at the range on the way to the match. We had a BC of .946 to get it to match drops at 600. Dope was solid to 1000 at the match. Obviously something in the AB setup was wrong but we couldn’t figure it out and neither could some of the other folks at the match.

Yeah, that’s not a thing (.946).

 

[Formidilosus]

Well that is a CRAPPY bullet. Based on my MV of 2900, that is 500y load in my 6.5CM. I am gonna have to get those DRT's to work it appears.

They are crappy in aero.

 

[Formidilosus]

This is what I came to ask about... What does a guy do in this situation? I know this is an extreme example, but you have a spread of .368 - .411 from one end to the other, so quite a bit in between to choose depending on your twist rate and MV.

In the terminal image of that bullet (1,800fps) the Mach 2.25 will run just fine.

 

[180ls1]

What difference in inches at what range? 20 points is ridiculously high and there is no way you are seeing a 20 point difference in actual BC if using Hornady’s BC for the bullets, and Berger’s for theirs.




Yeah, that’s not a thing (.946).

Are these likely scope issues?

 

[Formidilosus]

Are these likely scope issues?

Sometimes. Nearly always it’s a gun that isn’t actually zeroed.

 

[COPrecisionRifle]

This is what I came to ask about... What does a guy do in this situation? I know this is an extreme example, but you have a spread of .368 - .411 from one end to the other, so quite a bit in between to choose depending on your twist rate and MV.

The Hornady 2.25 Mach Value for a 147 ELDM is .351 and i have ran a .320 out of my last 2 NRL Hunter Rifles, a factory Bergara was .330, and my PRS rifle was a .330. (Those numbers won 3 matches and have 2 more top 3s) That matches up to the provided Mach 2.0 BC but the .351 is whats on the box. I ran the velocity off my Chrono and then trued the BC from there. As far as i know thats how everyone winning in PRS and NRL Hunter does it. I have found that trueing the BC is a more accurate way for all the rifles i have been around.