Trueing muzzle velocity vs BC

[waveslayer]

Individual barrels and barrel twist rates can affect bullet BC values, much more than than the 1-2% suggested by Bryan Litz.

I use LabRadar velocity decay to establish bullet BC from my barrels. Identical bullets fired in a different twist rate barrel, with barrels chambered with the identical chamber reamer, travel with BC value differences much greater than the 1-2% referenced in the above Litz sit down discussion. In my experience...

Once you've nailed down muzzle velocity, why would you tweak MV in your ballistics program? Why juggle MV in your ballistics program to match down range bullet POIs, after confirming your bullet's MV with LabRadar or Magnetospeed?

The culprit will be the error in BC input value. So tweak BC value. Not MV.

That's true to close ranges. The bullet doesn't lie, so tweaking BC too much is making up for velocity. Tweak velocity out to 600, no matter what the chrono says... your matching the calculator to your DOPE. Past 600-800 you tweak BC.

So not 100% accurate .

 

[nralifer]

Individual barrels and barrel twist rates can affect bullet BC values, much more than than the 1-2% suggested by Bryan Litz.

I use LabRadar velocity decay to establish bullet BC from my barrels. Identical bullets fired in a different twist rate barrel, with barrels chambered with the identical chamber reamer, travel with BC value differences much greater than the 1-2% referenced in the above Litz sit down discussion. In my experience...

Once you've nailed down muzzle velocity, why would you tweak MV in your ballistics program? Why juggle MV in your ballistics program to match down range bullet POIs, after confirming your bullet's MV with LabRadar or Magnetospeed?

The culprit will be the error in BC input value. So tweak BC value. Not MV.

I tend to agree with you. Why change an accurately measured value assuming the lab radar is the most accurate way of measuring the MV. What none of us have considered are the individual factors that affect the drag coefficient. These are air density, compressibility, viscosity, and the direction of the air flow (wind direction and speed) relative to the bullet. We forget that over extended ranges these atmospheric factors can vary significantly. All one has to do to appreciate this variability is to take a bike ride on a calm evening over about a mile. You will notice how the air can feel at times cooler and warmer as you pass through it. These are thermal columns of air due to ground heat. The warmer columns are less dense than the cooler air, so it is reasonable to believe the drag on the bullet constantly varies along its path. The high powered radar can measure changes in velocity over very short distances, so with the proper computing power a BC for each increment of velocity change can be calculated. A predicted trajectory can be calculated from data gathered over say 10 shots which is a best fit for the ten shots. Each shot is unique since there are a multiplicity of atmospheric variables constantly changing in between shots and from day to day. That is why "truing" the BC makes more sense.

 

[Pdvdh]

That's true to close ranges. The bullet doesn't lie, so tweaking BC too much is making up for velocity. Tweak velocity out to 600, no matter what the chrono says... your matching the calculator to your DOPE. Past 600-800 you tweak BC.

So not 100% accurate .

I hear you, but still see no need to adjust MV in addition to BC. Seems like you're adding a second, unnecessary fudge factor.

The purpose of BC value in a ballistics program is to accurately predict downrange bullet velocity over distance, which then enables the software program to predict accurate elevation and windage dope. Shouldn't have to enter inaccurate MVs into a ballistics program in order to get accurate predicted downrange bullet velocity. The proper BC value accomplishes that.

I input my known LabRadar MV and an "average" BC value that correctly predicts bullet POIs at say...., 1,000 yds. Or as far as I can set up, shoot, and measure bullet drop on my target backer. I understand the BC value of the bullet in flight does deviate slightly across that 1000yds as bullet velocity decreases. But the slight differences in BC value from the "average" value I input - across that 1,000 yd distance of bullet flight - doesn't cause enough error in predicted bullet velocity and flight to cause non-lethal hits on large game when dialing elevation dope at closer yardages.

 

[WiscGunner]

The simple reason that "truing" muzzle velocity is recommended in spite of accurate measuring instruments is because the mathematical model is inaccurately predicting the flight path. Plain and simple. Absolutely no way around it.

Now, does it work to shift the data in the model to misalign with reality a little so the end result is close enough in the middle range but off at close range and at far? …sure, for most people.
CDM are more like the old multiple "banded" BC method. Each band is a new hump in the theoretical arc course correct the model back inline with the actual arc of flight.

AB is NOT a very good predictive model and in-fact is no better the every other basic modeling app. Yes I said that. They have lots of fancy marketing and special buzz words and magic ways to calibrate it to be correct with incorrect data. Saying AB is a predictive modeling software is like saying Tesla is a car manufacturer. Both have a cult following yet neither is what the say on fave value. Tesla is a software company mapping roads and AB is historical modeling software that only gets "better" the more recorded real models are entered to offset the less accurate "predictions".

Not necessary with better math like Patagonia but there is a lot of effort, time and money invested in some of these popukar programs and that makes a very comprelling insentive to overlook the flaws.

Patagonia and Coldbore are actually able to make fairly accurate predictions with accurate data and are not heavily reliabt on a G1/G7 input. Vastly more accurate than standard 3DoF solvers like AB, BallisticsARC, Shooter, JBM, etc. tgat rely very heavily on the value input for BC. Hornady 4DoF doesn't use BC which is why they only list their bullets they have recorded drag curves thru Doppler.

But don't take my word for it as I am not an expert in this area, test it yourself. Enter the exact same data without so called "truing" into 3 different solvers Trasol (Coldbore math), Hornady 4DoF and any of the 3DoF versions listed above. Then launch some bullets at various distances out to real distance and compare actuals to predicted. Most people don't shoot far enough to see 3dof software fall aparts. Those that shoot much beyond 800-1000yrds will either need to start lying to their software, get different math or take lots of real world notes.



At the end of the day, there a lot of ways to get hits on target as long as you understand the strengths and yes weaknesses of your "system"

 

[Pdvdh]

I use Patagonia Ballistics ColdBore 1.0 ballistics software. After confirming my MV with LabRadar, I'll match a G7 BC to predict my longest range drops on target. Typically at around 900yds.

I then go hunting.

 

[nralifer]

I use Patagonia Ballistics ColdBore 1.0 ballistics software. After confirming my MV with LabRadar, I'll match a G7 BC to predict my longest range drops on target. Typically at around 900yds.

I then go hunting.

That makes more sense because there is some uncertainty in the G7 due to the rifle itself. What you are doing is matching the rifle's effect on the G7 of the bullets shot from that rifle with the muzzle velocity the rifle delivers.

 

[Parkerized]

Truing.

 

[Pdvdh]

That makes more sense because there is some uncertainty in the G7 due to the rifle itself. What you are doing is matching the rifle's effect on the G7 of the bullets shot from that rifle with the muzzle velocity the rifle delivers.

I also calculate the bullet's BC value from my barrel, using velocity decay over the first 100yds of bullet flight. LabRadar provides the velocity decay data. That calculated BC value is very close to true value over the first 100yds of bullet flight, as it's based on the average bullet velocity decay from multiple bullets fired during load development and future shooting events. If I fire 8, 10, or 12 bullets on one outing, I review the quality and consistency of the LabRadar data for each bullet fired. The velocity loss from 0-20yds, 20-40yds, 40-60yds, 60-80yds, and 80-100yds for each bullet fired is compared for consistency. Good quality data will show very similar velocity loss per 20yd increments of bullet flight. Then I'll average the recorded velocity loss for that group of bullets. Use that data to calculate my BC value based on my station conditions

After these steps, I find the tweaking of the bullet BC input value from my calculated value, necessary to produce correct long range bullet drop predictions from ColdBore 1.0, are relatively minor.

 

[nralifer]

I also calculate the bullet's BC value from my barrel, using velocity decay over the first 100yds of bullet flight. LabRadar provides the velocity decay data. That calculated BC value is very close to true value over the first 100yds of bullet flight, as it's based on the average bullet velocity decay from multiple bullets fired during load development and future shooting events. If I fire 8, 10, or 12 bullets on one outing, I review the quality and consistency of the LabRadar data for each bullet fired. The velocity loss from 0-20yds, 20-40yds, 40-60yds, 60-80yds, and 80-100yds for each bullet fired is compared for consistency. Good quality data will show very similar velocity loss per 20yd increments of bullet flight. Then I'll average the recorded velocity loss for that group of bullets. Use that data to calculate my BC value based on my station conditions

After these steps, I find the tweaking of the bullet BC input value from my calculated value, necessary to produce correct long range bullet drop predictions from ColdBore 1.0, are relatively minor.

I find also that the LabRadar G7BCs also correlate very well with ABs radar G7s as well. Currently looking at change in G7 between supersonic and subsonic velocity in 308 Win case as subsonic loads are easy to make.