Home Forums Hunting & Shooting Ask The Gunwriters Chronograph calibration & accuracy

Been pondering chronographs...I have a rifle that seems to run about 100-150 fps slow. Folks talk about slow barrels & fast barrels. I'm wondering how much of the variation might really be due to differences between chronographs. Many meters can be calibrated, but I've never heard of anyone calibrating a chronograph. Heck, how could you?

If I had more than one, I'd compare them. My inner Scot is keeping me from buying another when the cheaper route is to ask sage gunwriters and others if they've ever done a comparison. So, school me....

known loads help.

Sorry, not sure I get your point. Please forgive my dumbness. If you're looking for an example, here's one: .300WSM, 24" tube, 180gr Nosler BT, 67g RL22, Fed 210 primer, 2.84" OAL...a Shooting Chrony tells me it averages 2750 fps.

"reference ammo". think .22 LR, Ely Match or similar.

you need something you can send across that you can compare your reading to the published reference velocity(and believe that velocity).

you gotta pick one and go with it. Over time you'll develop your own "reference" loads -- ones you "know" the velocity of and can compare back to. Until then, you have to pick a (match) factory load an trust it.

Ah, I'm reading your mail now. If I shoot the reference load and it reads markedly different, though, how can I conclude the cause (gun vs. chrony) unless I can verify the reading against another meter?

Velocity is a derived quantity, distance divided by time. So you can't get a can of 1,000 FPS and open it up to check calibration. You can only check the accuracy of the timer and the accuracy of the distance.

The timing of the internal circuitry is the least of your difficulties. The internal oscillator is crystal controlled, and the accuracy and precision of even a very cheap crystal controlled time base is so small as to be insignificant.

One problem down.

The second source of error is the effective spacing of the photocells. If your screens are tilted, or the spacing of the photocells is off by just a little bit, it will make a difference. Ken Oehler says that maintaining better than about 1/8" precision is hard, and I believe that.

So that's one limit to accuracy. 1/8" out of 1 foot is about 1%. Not a huge factor, but not zero.

The third source of error is lighting conditions. If the light is diffuse, like you get on an overcast day, the shape of the electronic pulse you get from the photocells will be a little different from what you get on a sunny day. This will affect speed measurement. With the Chrony, if you leave the screens in place as a cloud passes over the sun, the readings definitely drop.

I solve that by only chronographing on days that are clear enough to see the shadow of the tripod, and only during the middle few hours of the day. BTW, on the Chrony, using the extension rods on the skyscreens drops velocities by about 25 FPS.

The idea of checking against something like a reliable .22 round is a good one. It's as close as you're going to come to a calibration in units of velocity.

Hope that long ramble helps somehow.

One other source of error Muzzle Blast. Back off another
10 feet and look for a change. Finally if the load is accurate, do you really care.
Good luck1

denton,

Nicely done explanation. Thanks.

Cheers Denton! If I'm understanding your post correctly, you're saying that, because of the very small potential for error in the "crystal-controlled internal oscillator", chronographs are inherently accurate and precise, so there's no substantial reason to believe that there will be differences in readings between any two units (all other things being equal). So, the standard error reported for a series of shots is entirely due to true variations in bullet speed, with no contribution from chronograph measurement error. That's what I'm wanting to hear, and if it can be taken to the bank, then a shooter can be assured that, so long as the chronys are set up and used properly, what they report is the true velocity of the bullet. Theoretically, that's all good. I still remain curious about whether anyone has set up two or more chronys and shot through them to compare the results (of course, accounting for the very small loss of speed over distance that would be expected).

As a sidebar, the conversation's also included the words "accuracy" and "precision". Interchangeably? You would think we shooters would be really anal about the clear use of those terms, yet we often talk about group size relative to accuracy, not precision. Precision is intrinsic group dispersion, while accuracy is the deviation from the intended point of impact, is it not?

denton - excellent points and well described. Sloppy measuring resulting in a minor distance error is probably the worst offender.

Use data obtained for your own load in another chrony. Use to check whether or not your own "drifts" from original. I've had to do it when a load shows way slower than book / anticipated results, kinda like when I have to decide whether or not a scope has shifted. Usually I find out nothing is wrong, with the equipment anyway. Weather and ME can be off on any old day.

Several times I shot over three different brands of chronographs on the same day. Sometimes shot each consecutively and sometimes I lined them all up in a row and had 1 bullet trip all the chronos.
The three brands were a ProChrono Digital, a Shooting Chrony Beta, and an Oehler 35.
Each time and in every test all three recorded similar velocities, IIRC well under 1% difference.
When the chronograph were lined up in a row I was frankly surprised by how closely the recorded velocities matched the predicted velocities based on distance from the muzzle.
My conclusion was that all three brands were accurately recording velocities and it increased the trust I have in the results I get with a chronograph.

probably not entirely. changing light conditions (especially from day to day) can play a big role.

Yes all barrels are fast and even the infamous P35 will lie to you!

Here's another one to use a "known load". It's a muzzleloader. I have a TC omega that I chonographed using 250 gr SST sabots and 777 pellets. My loads crongrographed within 10 fps of where they should be. I believe the minor variations in bore diamter that contribute to a fast or slow barrel are far less critical in a sabot gun rather than CF rifle with conventional bullets. Seems logical. This is based only a sample of one though. There was enough good data to cross reference and come up with an accurate estimate of velocity for that particular combination in TC Omega.

I have cronographed three other rifles and loads and they all were right where they should be. I'm using a ProChrono Digital FWIW.

edited to add: I'm partly basing my conclusions with the MZ on the fact there is no chamber, throat or barrel length variations to contend with. Sabots fit "loosely" in the bore, relatively speaking, so there's not much in the way of variations there either.

Yes it is. Good catch!

Electronic circuits tend to be a little microphonic. That is, some of the components will act as unwanted (and not very efficient) microphones. Ceramic capacitors are well known for this.

The flash of light that accompanies muzzle blast may also be a culprit.


Close. I think the correct statement would be that if the user is careful about setup, and if the geometry of the unit is correct, and if you limit measurements to days when lighting conditions are correct, the chronograph is both very accurate and very precise, enough so that the error is negligible. Add to that the requirement that you're not being tripped up by muzzle blast.


Yes. And when you get those pairs of numbers you can calculate the precision of the system. IIRC, the Effective Resolution of the Shooting Chrony (smallest difference you have a 50% chance of detecting) is around 1.3 FPS.


Not many are conscious of the difference between those two. Yes, what we usually call an accurate rifle is really a precise rifle. Accurate is when the average of many shots falls on the point of aim. Precise is when the shots are one ragged hole.

About ten years ago, I was building up loads for a rifle and there happened to be two other guys there with chronographs. As I recall someone was getting some strange measurements, so a couple of us decided to line up our chronies, then the third joined in. We were all kind of amazed how close they were in velocities (5 or 6 fps). Then we realized the first one was always the fastest and the back one was always the slowest. upon scrambling them the results were the same. At ten feet from the muzzle with a varmint bullet you are losing 4-6 fps over 4-5 feet of cronograph screens.

2 different brands of chronies by the way.

+1 lighting effects chronograph accuracy more than chronograph quality

I run a sample from a brick of 22 LR ammo that's been clocked at 1,230 fps over several units through mine. If it's within 10 fps I consider it good. I do that before every assessment just to make sure the unit's fine before lighting off my more expensive ammo. Will have to start over when that brick is done.

We lined my old Chrony and my son's new Chrony up and shot over them both. The differences were very small. What continually drives me nuts is the day-to-day variation that we see. Denton's explanation of the role played by the lighting conditions present each day is much appreciated and verification of what I have long suspected. I have mentioned before my wish for the availability of some sort of lighted tube through which to shoot that would create a more consistent level of light that wasn't affected by the amount of sunlight that is present day-to-day or shot-to-shot. Very good thread - thanks to everyone for their comments. Best to all, John

The accuracy of the crystal clocks is almost beyond question. Their frequency stability is typically better than the length stability of the stainless steel rule you might use to set the screen spacing. The problem with chronographs is not the stability or accuracy of the crystal oscillator, or even the counter circuits used to measure the short interval of time between start and stop signals.

The problem boils down to starting and stopping the timer at the right instant. Just where is the bullet when the start and stop signals are generated? The accuracy of the spacing between the screens is only part of the problem; you want to know the [precise] [accurate] distance that the bullet travels between start and stop signals. That is the distance you must assume when you convert time and and distance to velocity.

The best calibrations I can do is using lab type chronographs with a long base-line between screens and to then place the tested chronograph at the mid-point so that we can observe the velocity of the same shots. Purists will correctly protest that this is not a true calibration, but it is only a comparison. We agree, but then ask them to provide bullets of known velocity so that we can do a proper calibration. We haven't seen such bullets.

The 1/8 inch error mentioned by Denton was not intended to reflect only the error between screen measurements. The 1/8 inch assumed error reflects the uncertainty in the distance traveled by the bullet between the start and stop pulses generated by the screens. Even if you do an absolute perfect job of measuring screen spacing, you still have this uncertainty. (This assumes that you are using good skyscreens with proper lenses and properly illuminated diffusers.)

This 1/8 inch error corresponds to approximately one percent accuracy with a 12 inch spacing between screens. One percent corresponds to 30 fps at a velocity of 3000 fps. That is probably close enough if you are primarily interested in average velocities. If you are trying to measure uniformity of the velocities, then one percent accuracy (or precision) is inadequate. The only practical way to increase the accuracy of the measurement is to increase the baseline distance between screens.

Using a brick of .22 RF ammo fired in the same gun as a "reference" is indeed a good practice. It is not a calibration, but it will alert you to changes in the chronograph performance. It is also useful in troubleshooting you chronograph procedure. If the chronograph reads the proper velocity from the "reference" 22 ammo, yet doesn't read near the expected velocities of your test ammo, you must ask yourself how that dumb chronograph can tell which ammo you are firing.

Note that my comments are based on experience with Oehler chronographs. We won't presume to answer for others.

Ken O