Velocity Decay from Chronograph to Muzzle.

[DocUSMCRetired]

Muzzle Velocity Decay between Muzzle and Chronograph.

Did you know, the difference between your chronograph and the muzzle can make a sizable error in your firing solution? An error of 10 fps can equal an error of 5" or more at 1000 yards. More than one factor goes in to correcting for the distance between your muzzle, and your chronograph. Including atmospheric correction. All chronographs begin measuring the bullet, after it leaves the muzzle, so you must correct for the distance between. No chronograph is exempt from this, even the MagnetoSpeed is 1 foot away from the muzzle at the time of measurement. Chronographs measure velocity at their specific location, not Muzzle Velocity. To learn more check out this short article on adjusting for your Chronograph to Muzzle distance.

http://www.appliedballisticsllc.com/Articles/ABDOC121_VelocityDecay.pdf

If you have ever had a small error in your firing solution, knowing this simple bit of information can sometimes help you track down and remove that small error. This is just one of many small adjustments a shooter should check when the live fire, and the predicated solution don't line up.

 

[FearNoWind]

According to the chart, factoring in the 1 foot of travel using my Magnetospeed., my fps loss comes to less than .02. I've never found a MV calculator that will work with inputs having 1/4 inch variables. That's why I use the Magnetospeed.

 

[DocUSMCRetired]

According to the chart, factoring in the 1 foot of travel using my Magnetospeed., my fps loss comes to less than .02. I've never found a MV calculator that will work with inputs having 1/4 inch variables. That's why I use the Magnetospeed.

Not surprised, but that might not be the case for all users, under all conditions so its good to know. This is setup down to 1 foot variables, which is good enough in most cases. Down to 0.5 fps, and lets a user know if they need to address it or move on. One of the important things I want users to take away from this. Is that a Chronograph measures velocity at a specific location, not muzzle velocity.

 

[J E Custom]

I hadn't thought about the magneto speed being I foot off. I thought it would be half the distance (an average) from the first magnet to the last/second magnet (6")

When I use the Ohler, I would measure from the muzzle to the first screen and with a 4 foot spacing
and the proof screen I could determine the velocity loss in 4 feet and then add that x the distance and be close.

But I still make a drop chart to verify velocity and BC of the bullet.

In the future, I will add the one foot to the distance.

Thanks

J E CUSTOM

 

[Mikecr]

The Ohler is integrating the average of velocities between screens -instead of actual velocity at xx.x distance.
This is one factor I'm hoping LabRADAR will remove.
But for all I know, RADAR/it's triggering, could make back calculating to the muzzle more difficult..

 

[DocUSMCRetired]

The Ohler is integrating the average of velocities between screens -instead of actual velocity at xx.x distance.
This is one factor I'm hoping LabRADAR will remove.
But for all I know, RADAR/it's triggering, could make back calculating to the muzzle more difficult..

You can take the shot data and see how they are doing it with the Lab Radar, they are just using a linear path back to the muzzle. In the shot data, it will tell you the distance from the first registered ping on the bullet. In essence you can use the MV they give you, or you can do your own back calculation using the hard data provided in the shot file. In this post you will find an actual data file, I have plotted it so you can see what they are doing. The first point, was provided by the LR software, you can see the spacing between the first point at which the bullet was tracked, and the predicted MV of the LR.

For reference the shot was first picked up by the LR at 6.06 yards, with a velocity of 2584.02 fps, at 0.007021 seconds. I was using the Audio Trigger for this shot. You could use this data to do you own back tracking. In fact we can do this here. The G7 BC of this bullet is 0.282. This puts us at about 10 fps velocity decay at 6 yards. The LR estimated 2593.7. Using the math and BC we get 2594. Roughly 0.3 fps difference. Otherwise negligible in this case. So the LR did a very good job of lining up with the math in this case.

 

[Mikecr]

Thanks for the info.
How is it the LabRadar is plotting velocity decay as linear?

 

[DocUSMCRetired]

Thanks for the info.
How is it the LabRadar is plotting velocity decay as linear?

Only back to the muzzle. You can see the trend line. Everything else is measured.

 

[Mikecr]

Thanks Doc

 

[J E Custom]

The Ohler is integrating the average of velocities between screens -instead of actual velocity at xx.x distance.
This is one factor I'm hoping LabRADAR will remove.
But for all I know, RADAR/it's triggering, could make back calculating to the muzzle more difficult..

The velocity is measured from the first screen and then to the last screen And the proof screen velocity will be different than the final velocity showing the drop in velocity between the proof and the final velocity so I have been taking the velocity drop over the distance between the two sets of screens and using this for the velocity loss for that distance and adding the drop it would be for the distance between the barrel and the first screen and the difference in the two sets of screens and it has been very close to the muzzle velocity on the drop charts as long as the Bullet BCs are correct.

Don't know if this is the best way, but it has worked well in the past and when testing it against the magneto speed it was also very close.

Ultimately a drop chart is necessary in my opinion, to be able to forecast longer distance shot placements.

J E CUSTOM

 

[Greyfox]

Good information.
When using optical chronographs I have used my ballistic calculator to estimate the velocity gain to the muzzle. This has generally been 10-15 FPS depending on the particular caliber and distance of the chronograph to the muzzle. Ultimately though, I have always verified my drops at long range, and fine tuned either velocity(or BC) if it was necessary. The Magnetospeed and LabRadar that I currently use are close enough and probably within the degree of measurement error. While it has happened, I don't assume that my initial velocity inputs will result in perfect drop data.

 

[DocUSMCRetired]

Good information.
When using optical chronographs I have used my ballistic calculator to estimate the velocity gain to the muzzle. This has generally been 10-15 FPS depending on the particular caliber and distance of the chronograph to the muzzle. Ultimately though, I have always verified my drops at long range, and fine tuned either velocity(or BC) if it was necessary. The Magnetospeed and LabRadar that I currently use are close enough and probably within the degree of measurement error. While it has happened, I don't assume that my initial velocity inputs will result in perfect drop data.

We actually have an app that does this correctly. Accounting for BC etc. http://www.abmediaresources.com/ABToolbox.htm

 

[Greyfox]

We actually have an app that does this correctly. Accounting for BC etc. http://www.abmediaresources.com/ABToolbox.htm

Thanks Doc, I'll check it out.