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

Thank you Dr. Oehler.

I always appreciate your input into all things chronograph related.

NIST had a collection of those at one time, but someone opened the container and all the bullets were quickly lost.

Nice to see you, Ken.

Although I am a new member of this site, I am a long term lurker on this and several other shooting sites. This thread motivated me to register so I could counter some of the comments I have observed here and on other sites regarding the precision of chronograph readings. I have used a Shooting Chrony for at least five years, and based on my experience its only value is to compare velocities during load work ups on a given day. I have very little confidence in the actual velocities because of the variables and inaccuracies pointed out by Denton and Dr. Oehler.

I participated in a back to back comparison of three Shooting Chronys. About 10 shots of a given load were fired from one rifle over the three units. The difference between the highest and lowest readings was around 240-260 fps. The differences were quite consistent shot to shot. The highest velocity was recorded on the unit closest to the rifle and the lowest was on the middle unit. The third unit indicated about halfway between the highest and lowest. The average of the highest and lowest velocity for each shot was in the 2900-3000 fps range. The 250 fps difference is about 8.5% of the 2950 mean velocity. Based on these results, and other uncontrollable issues pointed out by Denton and Dr. Oehler I believe we should consider the absolute values recorded by our cheap Chronys with a few granules of NaCl(salt). On a given day average differences during load work up are useable if obvious wild readings are ignored.

Spot on.

Chronys vary as well with location and also the projectile. Checking a Chrony against an Ohler is meaningless as it only applies to that day/location and projectile.

Sounds to me like something was peculiar about that middle unit. Was the screen set-up the same on all three?

I've had a couple of Chronys, and found both of them to be consistent and to give results that always seemed correct. They have never given me a reading that I thought was 250 FPS low. I'm pretty careful to always do my measurements under a very standard set of conditions.

I've heard enough of these reports that I think there are two populations of Chronys, many that work just right and some that don't work well at all. I can also tell you that the people I can reach by phone at Chrony have no technical clue at all.

Having to do all my measurements outside on a sunny day is a real nuisance. I've figured out a much better design, which I can graft onto my existing Chrony, but so far it's just more work than I can fit in.

That is why I checked them on the same date, in the same location, and often with the same projectile.
All three were very consistent.

Bit of a roller coaster ride, this. Denton and Oehler were leading me to believe in my Chrony. Dbfletch's story gives me much doubt. I think the only way for me to sort this out is to take advantage of an opportunity to compare my Chrony readings, ideally against another unit and using a reference load.

Or...I could just buy an Oehler chronograph...

Wonderful discussion fellas. I came to the right place.

Folks, when Ken did his original chronograph it was quite an insightful and nicely done piece of work.

Once someone figures out the hard stuff, copying is easy.

Today, there are lots of good chronographs built on Ken's original work and principles. I think almost all of them work with all the precision and accuracy that any of us will ever need. If you're not getting screwy answers, chances are extremely good that the answers are accurate and precise to a percent or better. Note that there are reports from several shooters here that tested two or three chronographs in series and got very consistent results. That is what I would expect.

I'm not discounting dbfletch's report of the very odd middle reading at all. Obviously, it happened and it's not happy news. But in my experience it is the exception.

My rules are:

1. Protect the chronograph from muzzle blast. Usually 10 feet or so separation is sufficient. (Thanks for the catch!)

2. Make measurements only on days when the sun is distinct enough that you can see the shadow of the tripod. Do not attempt measurements at times when clouds cover the sun. I think the Oehler has better optics, but the Shooting Chrony definitely gives lower readings with the skyscreens in place when a cloud covers the sun. I know that with some units it's claimed, and probably with reason, that overcast days give more consistent results. I think you'll find more people that follow that than my suggestion. All I can report is what I've found with my equipment, which is that sunny days give better results.

Similarly, do not make measurements when the chronograph is in the shadow of a building or canopy.

3. On the Shooting Chrony, do not use the skyscreen extension rods.

4. Make sure you set up the chronograph properly and that the battery is fresh enough.

5. Shoot straight through the chronograph, not at an angle.

Not a rule, but possibly a factor: The range where I shoot only gives the option of shooting to the east. Maybe I don't have problems because the sun is never at right angles to the chronograph.

When I do all that, I always get very consistent and believable results with my Shooting Chrony.

The best thing about a "cheap" Chrony is when you shoot it they
will take it in trade. If you don't use the skyscreens and
rely on clouds for a background you will get bit sooner or
later.
God luck!

In 20+ years I've never destroyed any chrono by hitting it with a bullet or arrow.
Funny how many shot ones I've seen pictures of though.

Some interesting points being brought up here. I wonder if anyone can explain an odd occurrence that I've witnessed with my chronograph.

When shooting a string of shots over my chronograph using any one of my rifles and a single load recipe, I've witnessed the velocity of the successive shots fall progressively lower than the recorded velocity of the first shot. As an example, my first shot might register a velocity of 2,850 fps, the 2nd shot might be 2,830, the 3rd might be 2,815, the fourth 2,805.

I haven't tried to shoot a string of more than 3 or 4 rounds to see if recorded velocity continues dropping.

Am I the only one to ever experience this?

The screens were the same on all three units. Since the "third" unit recorded velocities halfway between the highest and lowest, it isn't logical to conclude the lowest readings were the ones in error. There was at least 4% difference between the "third unit and the two extremes. This comparison occured because the other two shooters were trying to develop loads to obtain a certain velocity with low ES and SD for 1000 yard competition. In addition to the three unit comparison, they fired their rifles over each other's Chrony and observed that the differences in measured velocities were attributable to the Chronys, not the rifles and loads.
The Chrony is marginally acceptable for load development comparisons on a given day. If one desires enough accuracy or precision to have faith in the absolute numbers a chronograph with longer screen spacing and a "proof" channel, such as the Oehler 35P is the one to use.

Ken Oehler;
I'd like to say a quick thank you to you and denton for the excellent and most insightful posts.

I've "known" I wasn't getting good readings on some days, but never put together the pieces of the puzzle as to why until now.

A tip of the hat to Matt as well for the suggestion of a control load. I happen to have a 100 pack of CCI Green Tag and and old Brno No. 4 that will do very nicely for that task.

Thanks again to you all and good luck on your upcoming hunts.

Regards,
Dwayne

Then as far as I can tell, you did everything right. I'm stumped. Perhaps you had a bad unit.

Addendum to previous rambling post: Common practice would be to use something like a Schmidt trigger to "square up" the bullet shadow pulses to start and stop the counter. In my mind, that should make the circuitry immune to variation in light. Nevertheless, lighting conditions do make a difference.

Wouldn't the old fashioned printed circuit paper screens provide nearly perfect and consistent start and stop pulses?
Super inconvenient and likely expensive but breaking a wire to to start and stop the timing circuit would be immune to lighting conditions.

Cheaper and easier would be to use artificial lighting like a LED array with the sensors shielded from ambient light. You'd have consistent lighting conditions. But I think ambient light is fine for most of us as a practical matter.

Denton, I traced the photo transistor conditioning circuit on my pro Chrono. It uses a rather clever but not unique circuit with discreet parts to keep the trigger pulse within range of the MCU port. Not exactly out of someone's app note that's around here somewhere but close. (Yup, repair job after wounding it. )

Paper screens have their own problems as do "shorting" foil screens. Doppler radar has its problems and spark photography is 'way too expensive. Using a Schmidt trigger to square up the screen response still doesn't tell you where the tip of the bullet is at the instant of trigger. (Parkinson has a law of "Compensating errors won't.") Until I learn better, I'll stick with the 1/8 inch uncertainty we've observed over the years. If someone says that 3/16 inch or 3/32 inch is a better guess, I won't quibble.

Let's just say that until you've used two or more chronographs to measure the velocities of the same shots, and then try to get the chronographs to agree under all conditions, you've missed much frustration. Measuring velocities is simple compared to ballistic coefficients and pressures, but it's all a challenging game.

Ken O

I bet!!

What struck me as a likely source of inaccuracy when repairing mine was tolerance differences of (discreet) components in the sensors and conditioning circuitry. If there's any factory calibration its hidden in the firmware.

Now the optics elude me.

That's probably where I'll end up.

There is a somewhat more elegant solution that I've dreamed up, but it may not be worth the effort. It would, however, give a nice repeatable input pulse.

3M makes tape with embedded corner cube reflectors. You've seen those.... directs light back to the source. They even have types "tuned" to various parts of the spectrum.

So instead of skyscreens, you make covers with the special tape on the bottom side.

Right next to your start and stop photocells, you place a laser diode, pointed up. Just above the diode you put a cylindrical lens to spread the laser beam into a slim fan. The special tape directs this back to the close neighborhood of the lens, i.e., the photocells. Then you slap a narrow bandwidth optical filter over the photocells.

Now you have a system that is pretty much immune to ambient light, and which will give very predictable response.

But, as I said, it's probably overkill. The LED array will probably work as well as anything there is.

Oh, hey, for overkill that's nothing. Dreamed up a scheme using microwave transceivers, the old Gunnplexers, to measure velocity by doppler. Sounded great until I got to the signal processing part though it might be doable with current DSP chips if you really know your onions.

You guys have clearly taken this to another level. It's kinda weird but I'm picturing you all shooting with tape on your glasses and wearing pocket protectors and camo hats.

Camo hats????


de W7DB, ex K7OWJ

Went metal frame on the glasses but has some kinks from playing with the dogs if that counts.

73, de KF0MD ex KE2KF, gotta sit for the extra if I ever seriously get back into it.

I was gone hunting for a few days, so missed getting in on the beginning of this one, but here are a few observations:

I got my first chronograph, from the Custom Chronograph Company in Washington state, in 1979. It had both breaking screens and light screens, and both provided the same readings. It worked very well but eventually broke down and the company was no longer around. It was pretty slow anyway, since it was the old style where you had to turn a switch and note which lights went on, then translate that into velocity from a table. But it was but was very consistent.

I then bought a Chrony and used it for quite a while until one day I was shooting a .41 Magnum revolver and forgot that one load with 170-grain bullets shoots a lot lower than my normal load with 220-grain bullets. The Chrony was 20 feet from the muzzle, since I was testing a .416 Rem. Magnum on the same day which always caused muzzle-blast problems, and I plunked the Chrony. It did a backflip, and had a hole all the way through the middle.

Replaced it with another, but soon shot it as well. However, this was only with a sabot from a shotgun slug load, which didn't actually put a hole in it. The Chrony people exchanged it for a new one.

So now I am on my 3rd Chrony, and it doesn't work as well as the first. (The second wasn't around long enough to come to any conclusions.) It gives readings that vary 5% or so depending on light conditions, and also misses quite a few shots. Unlike Denton's, it seems to do better on cloudy days, and that has been confirmed since I received my new Oehler 35 from the new batch Dr. Ken is making this year.

So far no shots have been missed by the 35, and I am really liking the proof channel. It isn't nearly as light-condition sensitive as any of my Chronys have been. I also suspect that the 4-foot rail makes precise alignment a necessity, which helps with consistent readings. Life is better.

PS--I have been "calibrating" my chronographs with the same batch of Remington .22 ammo, shot from the same .22 rifle, for many years now. Dunno if it helps or not.

I can't help but wonder if the replacement units they send out are units that someone else returned because they were unhappy with them.

It seems like there are two subgroups of Shooting Chronys. Most of them seem to work just fine. There seems to be another group that just won't behave.

infrared skyscreens and a 16 MHz clock. Available from NECO and expensive, but seems to take care of concerns expressed here. Light makes no difference, can be used in the dark.

I think the shooters location plays a part. Also, a lot of shooters just assume it is reading right.

I don't have any firm "statistical" type numbers but my feeling is the 375 H&H is more likley to read high on a Chrony than other calibres.

As a side note my first chroograph was the Ohler 10, about 1970-71 I think. In Australia at that time it cost about the same as 270 Sako

Just what I've always wanted to hear from someone. Have long suspected that this was the case. Thanks for the post.

If I were to market chronographs, I'd want them to read high. They would sell better because guys could brag about how fast their loads are and no one would be questioning if there was something wrong with their gun or their chronograph. Returns would be very rare.