Velocity Decay from Chronograph to Muzzle.

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DocUSMCRetired

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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. Remember Chronographs measure velocity at their exact location, not Muzzle Velocity.

Here is a great short article on adjusting for your Chronograph to Muzzle distance: http://www.appliedballisticsllc.com/Articles/ABDOC121_VelocityDecay.pdf

14317360_1182760501785831_5500185480188467584_n.png
 
One can also use the bullet maker's software to calculate muzzle velocity that reads the fps at chronograph range. I've done that with several cartridges with the screen centers between 10 and 20 feet down range.
 
Those who don't "true" their speeds by measuring drop a distant target from what your ballistic app calculated are probably going to miss first shots at LR. Many apps provide this feature.
 
Did you know, the difference between your chronograph and the muzzle can make a sizable error in your firing solution?

...even the MagnetoSpeed is 1 foot away from the muzzle at the time of measurement.
Click to expand...

Did you know, that if you have three chronographs over lapping at the same spot and fire a bullet through them you will have 3 different velocities?

There are also chronographs that calculate the speed from more than foot in distance. The Oehler 35 for example has 4 ft long sky screens.

I guess you could set up a Labradar and get some more data at least more than from a single distance. Even then you can just hope it's right.

A man with one chronograph always knows how fast his bullet is going, a man with two, is never sure.

https://www.shootingsoftware.com/doppler.htm
 
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After getting sight zeros at 100 yards and chronographing .308 Win loads with 155 and 190 grain bullets, then .30-.338 Mag loads with 190 and 200 grain bullets, along with noting atmospheric conditions, I ran the data on Sierra's software for each bullet's drop at 300, 600 and 1000 yards.

On the rifle range and making slight corrections for different atmospheric conditions, I shot those loads at the ranges mentioned setting the sights for calculated bullet drops at each.

At all ranges, actual drop versus calculated drop converted to MOA was less than 1/2 MOA for all ranges.
 
Just to clarify their are a number of reasons why the MV and your Dope don't line up. In some cases the corrected MV could be correct, but other factors unknown to the shooter are influencing the data.

1. Distance to Chronograph not calculated and accounted for.

2. Use of a poor, or unreliable chronograph. Read Here: http://www.appliedballisticsllc.com/Articles/ChronographChapter.pdf

3. SD of Ammunition is 15 or more (15 is minimum for good LR ammo, 20+ is junk).

4. Using the incorrect form factor (G1 instead of G7) Read Here: http://www.bergerbullets.com/form-factors-a-useful-analysis-tool/

5. You can negate form factor issues by just not using them, nothing is going to be more accurate than our CDMs: http://appliedballisticsllc.com/Articles/ABDOC130_CDM.pdf

6. Turret Tracking. Scope turrets are not always perfect, in fact a lot of shooters out there have turrets that do not track exactly. Here is how you can test them: http://appliedballisticsllc.com/Articles/TallTarget.pdf. Luckily turret tracking errors can be calibrated out with an MV calibration. When you do the Tall Target test, you have to measure, with a tape measure, from the turrets to the target. An LRF is not accurate enough.

7. TMOA vs SMOA. Not all scopes, ballistic apps & more are made the same. It is always good to double check is your scope working in TMOA or SMOA. Its also good to make sure the software you are using is working in the same units you are. 1" @ 100 yards (SMOA) is not the same as actual MOA 1.047" @ 100 yards.

8. Verifying Range. You would be surprised how inaccurate LRF devices can be. Especially some of the cheaper ones. We recently did some LRF testing, and the actual results, vs advertised results were disappointing in a lot of cases. A lot of LRF devices are good to +/- 3%, some are off by 5% and more. The further you go out, the more of an issue this is. Even with 1% accuracy at 1000 yards that's +/- 10 yards. 3% error at even 800 yards +/-24 yards. Its always good that a user verifies their LRF is providing accurate results. As you should do with all of your equipment.

9. Not accounting for DOF + Lat (Vertical Coriolis). This one is interesting, because its so easy to account for. If your shots seem high, check to see your DOF. If you are shooting east, and your firing solution is high by 4 inches at 700 yards. It might be VCor, and not actual error.

10. Aerodynamic Jump. Luckily we have this one figured out for you guys. But the number of times I have heard "I am off by a tenth of a mil, or a fifteenth of a mil" and they didn't account for AJ. It has become more well known lately, and our software accurately calculates it for you. But none the less, its there, and has been the cause of errors for shooters before.

11. Incorrectly doing a Ballistic Calibration. This one is very common, multiple times a day common. So I will just leave this here: http://www.appliedballisticsllc.com/Articles/BallisticCalibration.pdf

But, as you can see. A lot of things can manifest themselves, which can inaccurately pose a MV issue. Which can be corrected for with a slight adjustment to the MV.
 
so, school us on aerodynamic jump!

five inches @ 1,000 yards for a 10 fps variation, good reason to keep es numbers as low as possible.

kool post

murf
 
In my only test chronographing loads for a scope sighted rifle shooting prone at 1000 yards, I saw some bullets strike a couple inches above the call that chrono'd slower than other bullets leaving faster but striking a few inches below call.

There's a simple explanation for this. Anyone want to figure out why?
 
2. Use of a poor, or unreliable chronograph. Read Here: http://www.appliedballisticsllc.com/...aphChapter.pdf
Click to expand...

That's pretty much what I was getting to. They all give different readings, it would appear that some are so bad, distance from the muzzle being different by a few feet wouldn't make any difference at all, in accuracy or precision. For the ones that read consistently high (at least realitive to the Oehler used as a "standard") might be a better indication of actual muzzle velocity.

Would be interesting to have seen the Labradar in that group though.
 
I use my chronograph MV only to get me in the hunt the first go 'round so I don't have to do quite as much walking of shots onto target. My field card isn't based on the chronograph and the calculator, it's based on real-world, live-fire come-ups.

I also use my chronograph to test my temperature drift response during different times of year - I do my load work up, most of the time, in the summer where pressures are at their highest, then I check the powder response in the winter. I calibrate that against the same life-fire confirmation.
 
To answer BartB's rhetorical question; it's muzzle whip, or resonant vibration. Or, more simply put, the position of muzzle when the bullet exits the barrel.

But, to address the o.p. Title, my bullets leave the muzzle, and pass through the chronograph, not the opposite!
 
Assuming an accurate chronograph, and looking at it the other way, is the difference between 1000 yards and 3010 feet going to be 5" on the target?

It seems academic to me. If I chronograph my load and feed everything to a ballistics program and it puts me right on target today, the same load and rifle will probably call for a sight adjustment tomorrow.
 
I have read that bullets do not lose speed, or even gain some speed, for a short distance past the muzzle.

"As the bullet exits the muzzle the expanding powder gases continue to exert some force on the base of the bullet until the gas has dispersed enough so that the force on the bullet's base dissipates. This occurs, according to researchers at Aberdeen Proving Ground, at a distance of between 6 inches and 1 foot from the muzzle of a rifle. The velocity increase is very small--being only a couple of f/s. However, there is also another effect. Powder gases have a velocity of between 3,000 and 5,000 f/s on their own and the gas cloud actually passes the bullet and creates a cavitation effect which lowers the drag of the bullet while the bullet is in the gas cloud. (Similar to the ultra high speed torpedoes that envelop themselves is a cloud of bubbles.) Past the 1 foot max velocity distance from the muzzle the bullet actually has very little, if any, drop in velocity out to about 3 - 6 feet or so at the muzzle where the gas cloud has dissipated and thinned sufficiently that air drag can have an effect on the bullet."

gas.jpg

http://www.frfrogspad.com/miscellp.htm
 
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