Home Forums Hunting & Shooting Ask The Gunwriters Measuring BC

I've posted before on measuring Time of Flight to calculate BC. The best way to do it is with expensive radar ($100K+). The second best way to do it used to be with an Oehler System '88 ($18K). There is a more affordable option about to be available.

Dr. Oehler has created a new system, the '89. Like the '88, it measures velocity near the gun and TOF at whatever distance you want to test. Unlike the '88, it only allows one downrange measurement. Another big difference is the price. The '89 costs less than many scopes.

I've had the pleasure of testing the early models. There were some bugs that had to be worked out but I've had a functioning system for a few months.

Last week I had the opportunity to run it in simultaneous tests with an Infinition radar. The two systems agreed. The '89 gives velocity and a TOF measurement at one distance. The Infinition gives velocity and TOF over a continuous curve, until it loses the signal (~1200 yards on a 6.5 projectile, longer on bigger projectiles).

We took the '89 out to 3,529.5 yards. Two miles is 3,520 yards.

That's right, we were able to get measurements at TWO MILES. The radio signal indicated we could go farther. I think two miles is more than sufficient for my needs.

The System '89 is scheduled to retail for less than $2,500.00

The availability of affordable chronographs was a major advancement in long range shooting. The ability to measure TOF may prove to be just as big an advancement.

Bravo, Dr. Ken. Bravo, indeed!

I think that is pretty cool. Sometimes it is good to verify data. Be Well, RZ.

What am I missing?

Measure velocity at the muzzle with the rifle zeroed at 100 yards.
Shoot at range intervalls and measure actual bullet drop on target.
Plug data (velocity and drop at range) into ballistic calculator and tweak BC until match up...

This is a great advancement! The further the distance the more accurate the BC measurement

Since ballistic coefficient varies to some extent with velocity, what is most important is BC over the distance that shots will be taken. However, the distances that most shots are taken - 600 yards or less - could present some challenges for accurate and precise measurements due to the relatively short time of flight. Based on Oehler's reputation, I have no doubt the system is well thought out, and I'll be interested to see more info as it's available.

Your data is subject to variability in your groups.

BC isn't really a constant. The system that gives TOF as a continuous curve will allow for a better determination of BC along the flight of the bullet.

I just count backward from the birth of Christ.

The old ways (shooting at range intervals) still works but has been found to not be necessary. If we measure at far distance and predict at short, we are interpolating data. Measuring at close distance to predict far distance is extrapolating.

Interpolating the data has shown to be a very close match at all distances to the point where measurement was made and a little beyond.

If we measure at the distance where the bullet enters the transonic range (~ 1.2 times the speed of sound), the difference between using G1 and G7 is probably less than most of us could account for. In other words, the variance wouldn't cause a miss.

The test we shot used a bullet that is a good match with the G7. We knew we had radar data on it at long range. As the bullet is a good match to the G7, the BC shouldn't change much when going longer. Our test showed it didn't change at all.

The takeaway here is that the System '89 is giving accurate TOF measurements. Just like we use chronographs to find barrels/loads with the lowest velocity SD, we can now also sort for those that have the lowest drag (BC) SD.

And, it can do the measurements farther away than most people will likely ever shoot.

Exciting times, indeed.

The biggest challenge with measuring at shorter distances is precision of the distance measurement.

The '89 reports TOF to 5 decimal places. Timing is not an issue.

However, it is very easy to understand that a 1 yard measurement error at 100 yards is 10x more significant than the same measurement at 1,000 yards.

I had more confidence in laser range finders before I actually tested them. There are ways to improve their accuracy, such as a reflective target. Oehler has a modified LRF they sell with a reflective target. It gives a very good measurement. For shorter distances, there are laser measuring devices used in construction. I have a Bosch GLR825.

I suspect there will be many rimfire ELR shooters that will want to measure BC at 200 or 300 yards. I'd simply advise them to do the absolute best measurement they could and to drive pins (I use rebar) into the ground once they are satisfied with their points.

The precision of this system is best described as being controlled by how much you put into the setup.

mathman and BB, thank you very much for your explanations.

This sounds like just what I was wanting to see. Thanks

I thought the LabRadar unit does this stuff "on the fly".

Change in velocity vs. time.

You are correct. It has limited range, however, depending on projectile size and type.

I typically get readings from mine on .30 cal projectiles to about 100 yards. Last time I used it to track .22 rimfire, I think I got a track to about 50 yards.

I've used the Oehler '89 to measure TOF on rimfire ammo at 10x that distance.

That's what I do and it works very well!

Bob
www.bigbores.ca

So whos` bullet did you use with the good G-7 match? and will all bullets with that match fly as well?

As a consumer of various bullets with marketing hype attached IE, BC tested to distance with radar, is their data to be trusted? Or are you telling us this is a better system?

BufordBoone,
Thank you for sharing that. I remember corresponding with LarryB as he set out to measure BC's with Dr Oehler in Texas. There was 1000 yards of surveyors tape, 1000+ yards of cabling (along with "mice eating the cabling" problems). I decided back then that the preparations and challenges he faced (even with the support of a crew that included the inventor of the equipment used) were beyond my capabilities. ...and that was an improvement over what the brit Carmichael had employed (two sheets of foil board at 1K to act as start and stop gates to capture the 1K velocity). IIRC, Litz is interpolating BC's from two velocity captures (or is he using TOF?...time to drag out his book again!).

What you are describing sounds like it is one man deployable with better precision.

BTW, Infinition is the parent company of LabRadar.

The bullet we used is a large caliber, verified by radar to be a good match to the G7. By that, I mean the G7 numbers remained the same throughout various velocity ranges. Any bullet that matches a drag function (G1, G5, G6, G7, etc.) should be a match at all velocities.

So far as trusting BC tested to distance with radar, IF you test far enough, the radar is absolutely the best method. The '89 gives one downrange point. Radar gives a continuous curve. IF you have a radar system capable of recording to the transonic flight point, you would not need this system.

Wait, I should say IF you also have the ability to make use of the radar data. One of the most impressive things Bryan Litz has done is to make his radar measurements usable within minutes of recording. I remember when radar shots were "We will diagnose the data and get back to you. Probably in a couple of weeks:.

This system brings the ability to test at one range. If you are wise, you will choose the farthest range available to you. We verified that it worked at 2 miles. The cost is less than $2,500.

A note of caution. The 2-mile test was in very open terrain. Constricted terrain reasonably can be expected to diminish range. My range is probably about the worst of conditions (flat with trees on one side). I get reliable communication at 1,600 + yards.

The System '89 is wireless, battery powered and very portable. It is easily portable/deployable by one person.

So far as Bryan Litz, he is a man of Science. I'd trust his data as much as I'd trust my own. He has radar and is actually measuring projectile flight to come to the BCs he lists (In full disclosure, I'm honored to consider him a friend). In my best Southern accent: "If Bryan Litz told you a rooster dipped snuff, look under his wing and you'd find the box".

You are correct that Infinition is the parent company of LabRadar. The "Infinition" radar I referred to, however, is far more powerful than the LabRadar.

I'd have loved to participate in that test you mention with Dr. Oehler. He is a class act (and another friend).

On my bucket list is to one day hear "See that OLD MAN, he was friends with Dr. Oehler".

I`m sure you understood the numbers I was trying to varify were those provided by Bryan in his book. Thanks for the further explanation.

To calculate bullet BC's under the conditions you are shooting, I use a LabRadar and a Silver Mountain E-Target.

The LabRadar gives velocity at the muzzle and pre-selected intervals to 100 yards, the E-Target gives velocities at whatever distance the target is away from the muzzle, out to ~1000 yards.

Easy to calculate BC from these two velocities, easy to see if your scope movements are accurate, you can see (on your phone or tablet) your group develop in relation to your hold or wind conditions.

One thing that was realized right away is that the ES at the muzzle is not always the same at the target at 500 yards. This became an opportunity to analyze the QC on the bullets used.