Checking your own projectile BC

[Woody_rod]

For some time, I have been thinking about how to record my own BC data for projectiles, now that I have access to some custom projectiles. The BC is only estimated, so a much better idea of how these things travel needs to be known.

From what I have gathered, two things need to be known accurately:

1. MV, and
2. Time of flight

Using Litz' thinking on the "least reduction of flight time" as being the measure of best performance, the two previous sets of data are what is needed.

Given shooting say a Berger 180 gr 7mm VLD as a known quantity in BC and performance, this can be used as a benchmark for the new projectiles.

What I have been trying to figure out, is how to measure the TOF at 1000y for a true reading and cross reference to the known ballistics performance. I do realise that the atmospheric conditions should also be known, but if the known projectile is tested and recorded on the same day, then this can be the comparative data on which to base an educated guess of the BC. A percentage of difference is what the outcome will be.

Ok, so here is my idea to measure the TOF:

I downloaded an audio app for my laptop called "Audacity" which is free and works on my system as is. This program can record sound at decent quality and can also show it on a time graph.

Add two x handheld CB radios as commonly used at rifle ranges for the "microphone" at the 1000y end, and get set up at the range - one radio goes next to the laptop as the receiver, the other goes onto the target. I am thinking of taping it to the bottom of the frame with the mic pointed towards the target face (upwards).

The target puller / marker can then operate the target radio by taping the button down for a send signal, so the sound of the projectile going past will be sent back to the receiver and laptop mic to be recorded.

The laptop is situated next to the shooter on the mound and readied. The shot is fired - the sound will be clearly recorded at mound end, with the supersonic crack heard over the radio from the target, sent back to the firing point.

Some time loss will be evident (speed of sound across the distance to the microphone at the target end and the shooter end), BUT, if the same setup is used for all testing, including that for the known projectile, then the real data collected will still be useful.

Some very small time loss will be for the 1000y and radio comms, but this is so small as to be of no importance.

I hope to trial this shortly to test the custom projectiles I was talking about previously.

[Norm]

I think it is possible to set up some of the new type electronic targets so that they can measure TOF.
The MV would be easy enough so long as your chronograph is accurate.
The result will be an average BC over the distance shot given the starting velocity. If you are working on a G7 drag profile it should be good enough for most F-class shooting for dial ups. If you are trying to work out the BC for retained velocity and wind drift, then you would definitely need the second chronograph at the target end IMHO.
You would need to be a good shot, but it would provide valuable data. This could end up being expensive though if you don't provide protection for the chronograph. I use a steel deflection guard on my chrony holder and it works well but it would not be safe to have an observer near by so remote recording would have to be done.

[Woody_rod]

Using the data from the known projectile, and then reverse engineering the MV or V loss, a cross comparison should be pretty straight forward.

Using this method, I computed my 260 wildcat elevtion within 0.5MOA at 1000y. In other words, the terminal velocity (TV) is also constant, and does not need to be known, because the other constant (time) will tell you what it will be.

[AlanF]

Rod,

Hmm, looks like I've been beaten to the punch - but here's my two cents worth anyway.

Another way of calculating BC is using MV and difference in bullet drop between two ranges. To avoid the effects of changing conditions, best results would be obtained by having targets at two distances, one at approx twice the other (e.g. 300 and 600 yards), and alternating shots between them to form two groups. The difference in elevation is the key figure. To avoid inaccuracy in scope mechanicals, the scope elevation could be left at one setting, and elevations deduced by measuring from aiming point to centre of group on each target. These can be converted to MOA, and subtracted one from the other. Any good ballistics program can then be used for the calculation. You take an initial guess at the BC and using the average MV, see what the elevation difference between the two test ranges SHOULD be. Then its just a matter of making small changes to BC until the calculated elevation difference is exactly the same as the measured elevation difference. The final BC should be the average for the projectiles used. For this it would of course be better to use G7 BC rather than G1, because G7 fits our typical velocity curve better.

If you did want to persevere with the TOF determination method, I don't think it'll be long before some of the ET manufacturers start measuring this very accurately. The Ozscore system has muzzle blast detecters, so the "microphones" are already there at each end. However I'm not sure whether the comms technology between ends will allow sufficient accuracy yet.

Alan

[Woody_rod]

Alan,

I did think of this also - actual drop figures. The major problem is atmospheric conditions. Using time as a constant take that out of the equation, although the two things will theoretically give the same outcome.

By testing alongside a very well know set of BC data (the Berger 180 VLD), the rest can be very closely estimated within a few percent accuracy for the custom projectile.

The time method provides a way of estimating drop and windage on any range or condition, and should be able to provide the information by calculation. This is the main purpose of the test - to be able to get BC numbers, not what the drop is.

[Norm]

Using the data from the known projectile, and then reverse engineering the MV or V loss, a cross comparison should be pretty straight forward.

Using this method, I computed my 260 wildcat elevtion within 0.5MOA at 1000y. In other words, the terminal velocity (TV) is also constant, and does not need to be known, because the other constant (time) will tell you what it will be.

That's not quite how it works.
BC changes with velocity. So you need to know the velocity at the start and at the target as well as the TOF.
I will give you three examples of what I mean.
First projectile starts out at 2850 fps and looses velocity at a constant rate and travels 1000 yards in 1.80 seconds.
The second projectile starts out at 2850 fps and looses very little velocity over the first 500 yards but then sheds velocity quickly over the last 500 yards. It gets to the target in 1.80 seconds.
The third projectile sheds velocity quite a bit over the first 500 yards but then has less drag over the last 500 yards and so reaches the target in 1.80 seconds.
So the apparent BC of all projectiles is the same based on your calculations but in fact they have greatly different BC's depending on the velocity at the time and distance shot over.
Your method should be ok for a single MV and distance, but change the MV from one calibre/rifle combination to another and shoot over a different distance and some variation might appear. Especially at longer distances than 1000 yards.

[AlanF]

...BC changes with velocity. So you need to know the velocity at the start and at the target as well as the TOF...

I agree with you Norm, BUT the G7 BC changes much less with velocity than the more common G1 BC as used by most bullet manufacturers. In fact, I would say that G7 BC calculations would actually allow you to calculate a velocity at the target quite accurately from the TOF and MV alone. If you also had a measured velocity at the target it would give a more credible result, but I think muzzle velocity alone would still be enough for good comparisons between the high BC target projectiles we use.

That's just my opinion of course. A pity we don't have Bryan Litz as a forum member. He does post on Longrange.com occasionally. I might ask the question there.

Alan

[Woody_rod]

Norm, I understand how this works, I have been trying to include everyone in the thread so they can understand it.

There is no need to know the V at the target, this can be worked out by use of the known example BC. The TOF will give the TV - they are linked together.

I will put money on the fact that if: given the same MV, if one projectile takes longer to get to the target than the other, it will have a lower BC by around the difference in TOF - as a rough guide. BC unless tested accurately is only ever a guide anyway.

One could also work out what the air density is, if the other aspects are accurately known.

Each projectile model will pretty much always display the same outcomes, based on the MV and the distance. They cannot change the way they behave as they have physical elements that are fixed:

- Drag
- Mass
- Profile
- Balance
- Etc.

No matter what rifle you use, the MV from one, if it is the same as another, will give pretty much the same outcome.

[Woody_rod]

Maybe we can call the BC difference - "the shedding of V differential".

The difference of TV is really the only thing that matters in this discussion. If two very different projectile designs get to the target at the same time, in the same conditions, at the same MV: then they will have the same BC. If one gets to the target faster, it has a higher BC - simple.

There is no need to even work the numbers.

The BC numbers are one thing, but actual shooting on a range, using the actual conditions and MV being achieved are the real outcome.

[Norm]

Alan,
I agree with what you are saying. It would be within the accuracy required for this exercise using G7 BC's.
I have spoken to Bryan about this and a heap of other ELR stuff and it is interesting that even with all the technology that Bryan has at his disposal he still gets quite a bit of variation in his results. That is why he lists the batch number with all of his tests.
Others that have done tests with equal levels of technology and gear have also come up with different results to bryan with a lot of the same projectiles that were tested in common.
Bryan's tests were conducted over distances out to 1000 yards and are only valid to that distance. Past that his results are based on assumption.
I have shot a number of the projectiles he tested. Some out to 2500 yards and I can tell you that the G7 drag profile starts to fail past 1000 yards.
However for this exercise as a direct projectile comparison out to 1k then go for it. BC testing is a lot of fun. But there is more to it, the deeper you get into it.

[Woody_rod]

This is why I want to test my projectiles - so I have hard data to go on - not subjective heresay.

As long as we only shoot to 1000y in competition, it is not important what happens after that.

[AlanF]

...the G7 drag profile starts to fail past 1000 yards...

I seem to remember reading a disclaimer (from Litz?) that funny things happen around mach 1, and I presume your 2500yd testing would be well and truly subsonic Norm?

[Quick]

Norm,

What BC value would you use after 1000yds? I know some ELR guys who use the G1 BC after 1000yds or similar.

This is a very interesting discussion, please continue.

[Woody_rod]

Hijacked by pointless ELR noise.....

[OuttaAmmo]

What can you tell us about these custom projectiles?