Labradar users: Normal for Velocity SD to increase with distance?

[JimGnitecki]

Any of you shooters who use a Labradar for velocity measurment at both the muzzle AND downrange at different distances:

Is it normal for the standard deviation in the downrange velocities to INCREASE with distance from the muzzle?

I am asking because with my Cowboy Action 357 Magnum rifle and handgun load, the SD stays pretty level at CAS ranges. But with my Pedersoli Sharps replica current handload, the SD increases rather dramatically with distance from the muzzle:



I have noticed that at the 150 meter = 164 yard range that I am currently doing test shooting at, the best 5-shot groups seem unable to get below 1.5 inches (about 0.87 MOA at that distance). I am wondering if this climb in SD is causing that, and if so, what could be causing the climb in SD?

Jim G

 

[Blue68f100]

There as just a thread that discussed this. Labradar said to discard any reading past 100 yrds.

 

[JimGnitecki]

There as just a thread that discussed this. Labradar said to discard any reading past 100 yrds.

No. Not so. My Labradar does provide readings, consistently, beyond 100 yards when firing the big .45-70 500g bullet whose diameter is so large. Yes, the smaller you go in caliber, the shorter the distance that the Labradar can reliably get a reading. You start to get no entries in the .csvb file for that speciifc distances you have requested when that happens. But no, it does NOT automatically stop trying to take readings - at ANY distance you have requested.

And besides, this is NOT a failure to report readings. The issue is that the standard deviations in the velocities at different distances is changing. Since the velocities are decreasing with distance (because the bullet is slowing down due to aerodynamic resistance), you would expect the standard deviation (i.e. the relative variability) to either stay the same OR get lower in absolute number values (since the velocity is a smaller number as distance increases). You would NOT expect SD to get LARGER unless the bullets are experiencing some effect that is affecting each one DIFFERENTLY.

Jim G

 

[ballman6711]

I think you are trying to use the unit beyond it's design parameters. Kind of like a car tire, exceed the weight rating or the speed rating and it may work, but what kind of results are you going to get?

Is it normal for the standard deviation in the downrange velocities to INCREASE with distance from the muzzle?

There as just a thread that discussed this. Labradar said to discard any reading past 100 yrds.

Your SD is going to be tied to your velocity readings, and if the velocity readings are incorrect then your SD readings will also be incorrect. You yourself said that Labradar told you the unit wasn't designed to read velocity past about 100 yards, so you shouldn't expect accurate readings past that distance.

Yes, it may pick up that great big bullet, but just like overloading your tires, your results are going to be less than optimal, and possibly way out there.

chris

 

[MEHavey]

I'm going to offer that velocity Doppler-radar readings are not going to degrade so much as
the actual velocities will be increasingly affected by barometrics, subtle (cast) shape differences,
drag from yaw/precession differences, and consolidated BC differences between individual
bullets as they are in longer & longer flight.

So yes... I would expect downrange velocities to increasingly diverge as errors pile up..

 

[jmorris]

No. Not so. My Labradar does provide readings, consistently, beyond 100 yards when firing the big .45-70 500g bullet whose diameter is so large. Yes, the smaller you go in caliber, the shorter the distance that the Labradar can reliably get a reading. You start to get no entries in the .csvb file for that speciifc distances you have requested when that happens. But no, it does NOT automatically stop trying to take readings - at ANY distance you have requested.

From page 20 of the lab radar manual.



I would only expect things to get worse as you approach the limits of what it can do.

I have a Chinese made caliper that has a half a thousandth resolution but it’s not that accurate…

 

[Legionnaire]

If we assume your distant readings are accurate, you are likely seeing the effect of differential drag on low BC projectiles. They won't lose velocity as consistently as a high BC bullet.

 

[JimGnitecki]

I am inclined to agree that I am pushing the capabilities of the Labradar with this.

Jim G