Computing BCs with Labradar Velocities

[Berger.Fan222]

https://www.shootingsoftware.com/doppler.htm

Over the years I've suspected BC depends on the rifle a bullet is shot from, and I have read some articles to that effect also. Reading the above article at the Shooting Software site gave me the idea to use near and far velocities measured with the Labradar, together with atmospheric data from my Kestrel, to determine some BCs from the rifles actually being used. This approach also allows determination of a G7 BC (if appropriate) if one is not available for a bullet.

So far, the approach seems to be working. The G7 BCs I've gotten for the 168 tipped TSX and 208 Hornady ELD agree well with the numbers from Applied Ballistics and Hornady. On the other hand, as I suspected it might be, the BC for the 125 grain SST is higher than Hornady suggests at 0.330; whereas, the BC for the 110 grain V-MAX is lower than Hornady suggests at 0.245. The BC of the 55 V-MAX (0.224") is a bit higher than Hornady suggests in one rifle, but right on in another.

This seems like a good approach to determine ballistic coefficients in your own rifles.

 

[rardoin]

As Labradar's documentation suggests the larger projectile will likely read more accurate than a smaller x-section projectile so i would expect potential variation in readings on a smaller caliber. It is interesting that you correlated with the published data on the 168gr. What was the furthest distance downrange that the reading were acquired? I was thinking of doing the same calculations with my Labradar but figured with a 6.5mm boatail I would likely lose acquisition at 125yds and would not think that the data for such a short flight time would be of value. If it proves to be accurate that would be a good aid for determining the BC of custom bullets.

 

[Bryan Litz Ballistics]

I spoke with the labradar tech about this very thing and he advised against it. The unit doesn't track far enough to get reliably accurate BC's for use in long range shooting. I also asked about putting labradars downrange and measuring retained velocity. He stated the units aren't intended for this application and there are several reasons that preclude them being used this way.

Believe me I wanted to do it, but I'll pass on the advice from the tech which is: "it's not what they're designed to do".

They are very good at their intended purpose, which is accurately measuring muzzle velocities.

-Bryan

 

[Berger.Fan222]

As Labradar's documentation suggests the larger projectile will likely read more accurate than a smaller x-section projectile so i would expect potential variation in readings on a smaller caliber. It is interesting that you correlated with the published data on the 168gr. What was the furthest distance downrange that the reading were acquired? I was thinking of doing the same calculations with my Labradar but figured with a 6.5mm boatail I would likely lose acquisition at 125yds and would not think that the data for such a short flight time would be of value. If it proves to be accurate that would be a good aid for determining the BC of custom bullets.

Since one can use near and far data to compute G7 ballistic coefficients, I think the numbers for a given rifle can be meaningful for boat tail bullets.

The range display settings effect how far the radar attempts to read, so I have not set it up to read past 100 yards yet. However, The signal to noise was very good on the 55 V-MAX out to 80 yards.

My experience so far suggests that the unit is more sensitive to alignment than bullet size. I get it aligned first with 30 cal bullets until it is reading reliably at the furthest range I have it set for. Then I switch to 22 cal bullets and do a finer tweak in the alignment until it reads those out to the furthest range I have it set up for.

I tend to think the unit is accurate, because when I compute BCs for the same bullet for 20 different shots, the resulting numbers are very consistent when things are well aligned: standard deviations close to 0.5%.

I think it is an open question whether it will be better to use a BC value measured over 600 yards with a different rifle and load, or a BC value measured over 100 yards or so with the exact same load you are using. If there is a big difference between your value over the first 100 yards and someone else's value over a similar distance, it would make more sense to me to use the value measured with the same rifle.

For example, in the rifle tested, the 55 V-MAX has a higher BC than the 53 V-MAX, which may seem unexpected, because Hornady lists the BC of the 53 V-MAX at 14% higher. I had long suspected this to be the case from many years of shooting out to 600 yards (less drop and wind drift for the 55 V-MAX), and now I know why.