A tale of two chrony's

[Mike6158]

I hauled my trusty old Shooting Chrony Beta and my shiny new CED M2 out to my brother-in-law's place along with a Sendero SFII in 7 mag. The Sendero has a Jewell trigger set at 1# 5oz, a KDF Accuracy muzzle break, and a Harris BR-2 bipod outfitted with a Pod-Lok (the Pod-Lok is a nice little accessory). Other than that it's dead stock. I had 10 rounds loaded .010 off of the lands with RE-22. The load data came from the Sierra reloading manual but the bullets were 168 gr Berger VLD's. Per the manual, velocity (for a 168 gr SMK) should have been 2,950 fps. Range was 600 yards with a slight (4-5 mph) 45 degree l-r tailwind. I was using LB 3.0 for initial and followup settings for the scope. The scope is a Leupold Mark IV 8.5-25 x 50mm LR/T. I think that covers the setup.

I set both chrony's up in series, SCB first, at 10' - 12' from the muzzle. I don't have a good spotting scope so I was shooting three and walking down to the target to see how things looked (I got my exercise today). The first three shots were calculated using the book value of 2,950 fps in LoadBase 3.0 (along with actual wind, baro, etc). Going from memory I believe my initial value was 11.75 moa up (100 yard zero)

The chrony's disagreed with one another by about 100 fps. I've always trusted my good old, solid metal Shooting Chrony and it gave me velocities just under the book value of 2,950 (that rarely happens to me btw) so my initial thought concerning the CED M2 wasn't too good. I walked down to the target to mark the hits. They were low by a good bit. 2 MOA-ish. Ruh roh... it was looking like my SCB was wrong and the new plastic el cheapo looking CED M2 was right or at least closer to right. So... I plugged in the lower velocity info from the CED M2, dialed the scope up 2 MOA per LB3's results, and fired 3 more. This time I was about 2 cllcks (1/2 MOA) high and I needed some wind correction. About 3 clicks if I remember right. Winds were kind of tough (I'm just starting to deal with reading wind seriously) because of a row of trees parallel and upwind of the bullet path plus they were variable from direction and velocity (0 - 5 mph). Downrange the mirage would lay flat at times... others it would be nearly vertical. Follow up shots were within an inch or less of the aim point.

This is my viewpoint of the chrony situation. I think they are both good pieces of gear considering what they cost. I prefer the construction of the Shooting Chrony. I like the big readout of the CED M2. Obviously I prefer the better data that comes from the CED M2. However- based on bullet drop, I don't think I would bet any money on the validity of it or the SCB's data. Chrony's, imho, at least the lower end ones like these two models, are good for determining SD and ES but the velocity data doesn't seem to me to be on. That said I also have to throw in the possibility that the scope calibration is off. Ie 52 clicks not being equal to 13 moa. So if the scope is off a little and I didn't input something just right in the software, especially considering I had a slight, variable, quartering crosswind, some of, if not all of, the "error" could be attributed to the software and scope rather than the chrony. My guess is that they are all off to some degree.

On another note, the worst group that I had at 600 yards was 6.63" and the best was 2.8". The first group was the worst one and the last group was the best one. The middle group was about 4". I think that, as evening came, the winds died down and that helped the groups out a lot. That and the shooter (me) finally got in the groove and started using good trigger control.

The bottom line for me is that, just like in my work, if you have two measuring devices that don't agree then you have to use an outside method to determine which one is right and which one is wrong. An outside method in the case of my two chrony's was the bullet drop vs velocity data that I had available to use.

 

[Pdvdh]

I think you need to determine your scope's click values before using the number of clicks required to match LoadBase predictions as an effort to determine MV or which Chronograph is reading correctly or incorrectly. Even then, chronographed velocities are the way to determine MV. Not measured bullet drops. Scope click values are something that can be precisely determined. Once known, you can enter the exact value into LoadBase - which removes one source of potential error from your analysis of the problem.

Clearly something is up with one or both of the chronographs if readings differ by 100 fps. Now you need a third one to add to the two so you can figure out which one gives the bogus readings. Well... seriously, maybe you know someone that has a chronograph that you could borrow for one shooting session. That's what I'd look at doing next. And if you buy a third one, look for a used Oehler model 33 or 35. My Oehler seems to be pretty reliable.

 

[Mike6158]

I think you need to determine your scope's click values before using the number of clicks required to match LoadBase predictions as an effort to determine MV or which Chronograph is reading correctly or incorrectly. Even then, chronographed velocities are the way to determine MV. Not measured bullet drops. Scope click values are something that can be precisely determined. Once known, you can enter the exact value into LoadBase - which removes one source of potential error from your analysis of the problem.

Clearly something is up with one or both of the chronographs if readings differ by 100 fps. Now you need a third one to add to the two so you can figure out which one gives the bogus readings. Well... seriously, maybe you know someone that has a chronograph that you could borrow for one shooting session. That's what I'd look at doing next. And if you buy a third one, look for a used Oehler model 33 or 35. My Oehler seems to be pretty reliable.

After a while I could accumulate enough chronographs to cover a full 1,000 yards

My Shooting Chrony Beta is at least 5 years old. Probably closer to 10. The CED M2 arrived on Santa's sleigh this year so it didn't really surprise me that it seems to be closer to correct. Oehler... I looked... and looked... and looked but none were to be had right now.

I plan to back up and "check my clicks" on the next go-round. The ES values on the loads that I've tried are pretty high. Certainly not single digit. 30-ish to 60-ish. So I still have a lot of work to do in that department. RE-22 looks promising. Onward through the fog.

 

[Jon A]

Interesting data. Thanks for posting. It's a good "reality check" for people. I think there are a lot of people out there who don't realize their chronograph is off by a pretty decent amount. Then when they find the ballistics programs don't match they go messing with the bullet's BC in the program when that should be the last thing on the list....

 

[lisagrantb]

I did the same thing about a week ago. I compared a Chroney Beta and a discontinued three screen Oehler, the old three screen model suppose to be very accurate. We set them up in series and shot five rounds. The SC was about 130 fps faster, that is what I was expecting from the cheep Chroney. I started looking at the Chroney and found that the front screen wasn't all the way against the stops, I tightened the clamp that held the screen and verified exactly 1 foot spacing between the two eyes. We shot three more 5 shot groups and the average velocity for each group was within 2 FPS of each other. So I guess I better take back that comment about the Chroney being cheep.

 

[Mike6158]

The Shooting Chrony is about 1/5 the cost of the Oehler... but I wish that I had bought the Oehler when it was available. They go "used" for over $600.00. At least that's what I've seen when I googled Oehler 35 and come across expired auctions.

 

[MontanaRifleman]

Interesting post Mike. I've run across the same thing. My two chrony's being off anywhere from 5 to 50 fps. I dont think they are even good for SD's or ES's. They might distinguish higher ES's from lower ES's to some degree, but I take them with a big grain of salt.

I'm not sure a third chrony would tell you which is right, unless it was a really good one like an Oehler. If you had three ordinary chrony's you would likely have three inaccurate chrony's. And I've seen them act different on different days.

How sure where you of your zero, which I'm assuming you based your drops off? I've read the Leupy's are IPHY and not true MOA, so as already sadi, you need to find out just what your turret is calibrated for.

Drops aren't the most accurate way of measuring, but if you shoot them at regular intervals and measure them, I think you can graph them and get a fairly accurate trajctory to calcualte a velocity off. Just thinking out loud...

 

[LouBoyd]

All electronic chronographs have a crystal controlled time base. Those rarely go wrong and can generally be counted on to have an accuracy of 1 part in 100,000 or better. Most of the commercial chronographs have a very short measurement base. If you have a 100 fps error at 3000 fps thats a 3% difference, If the chronograph has an 18" measurement base thats only 0.54 inches bullet travel error which is shorter than the length most bullets. A difference in the lighting on the two screens can cause that much error. The sensor is trying to detect a very tiny drop in the light coming from a fan shaped area above the sensor. If one sensor is triggering on the meplat and the other is triggering on the (larger) shank of the bullet thats over a 100 fps velocity error on a typical 7mm VLD.

I can't tell you where the 100 fps error is coming from between your two units,. It's possibly the sensor spacing but more likely to be a sensitivity difference in one of the sensors. That could be from a difference in lighting (misaligned sky screen?) , contamination on a len (dust?), a difference in shot placement over the sensors(unit tilted relative to the trajectory?), or maybe just a manufacturing difference in the sensors. It's possible but very unlikely to be in the electronics.

I've used an Oehler model 35 for many years. It came from the factory with a 2 foot spacing rod for the sensors. Thats 1 foot between sensors. After a few sessions I made an 8 foot spacer and that improved my handload's measured SDs noticably. That's not to say the Oehler is a bad unit, only that you can't make reliable velocity measurements with a 1 foot measurement base unless the sensors and their alignmet are near perfect. Its also easier to make good measurement on stubby wadcutters than on long tapered VLDs.

 

[Mike6158]

All electronic chronographs have a crystal controlled time base. Those rarely go wrong and can generally be counted on to have an accuracy of 1 part in 100,000 or better. Most of the commercial chronographs have a very short measurement base. If you have a 100 fps error at 3000 fps thats a 3% difference, If the chronograph has an 18" measurement base thats only 0.54 inches bullet travel error which is shorter than the length most bullets. A difference in the lighting on the two screens can cause that much error. The sensor is trying to detect a very tiny drop in the light coming from a fan shaped area above the sensor. If one sensor is triggering on the meplat and the other is triggering on the (larger) shank of the bullet thats over a 100 fps velocity error on a typical 7mm VLD.

I can't tell you where the 100 fps error is coming from between your two units,. It's possibly the sensor spacing but more likely to be a sensitivity difference in one of the sensors. That could be from a difference in lighting (misaligned sky screen?) , contamination on a len (dust?), a difference in shot placement over the sensors(unit tilted relative to the trajectory?), or maybe just a manufacturing difference in the sensors. It's possible but very unlikely to be in the electronics.

I've used an Oehler model 35 for many years. It came from the factory with a 2 foot spacing rod for the sensors. That's 1 foot between sensors. After a few sessions I made an 8 foot spacer and that improved my hand load's measured SD's noticeably. That's not to say the Oehler is a bad unit, only that you can't make reliable velocity measurements with a 1 foot measurement base unless the sensors and their alignment are near perfect. Its also easier to make good measurement on stubby wad cutters than on long tapered VLDs.

You make very good points Lou. I have, but didn't use, IR sky screens for both Chrony's. I'll test again with those. If nothing else that will give me consistency of lighting.

I wonder if I could use a longer rod with the CED M2? Calibration would be off (obviously) but it might be worth sending an email to CED to see if they could do something like that. That's not an option for the Shooting Chrony due to the way it's made.

 

[Mikecr]

What we need is a chronograph that can be calibrated in the field to a primary standard of speed. I believe that comes down to MACH (using mics).

Temp& Rh sensing (to derive mach) would need to be built in, and a new system R&D'd from scratch. Not all that difficult but:
-There would have to be a way to keep counterfeits/copies & diminished consumer loyalties (as we see with Farley rests, Harris bipods, etc) from undermining reward of effort.
-It would have to be built and market dumped quicker than every single part goes obsolete(cheap -disposable junk- in final form).

Can't see it happening, but when China/Walmart finally does it, you better jump on it.

 

[Pdvdh]

I sent an e-mail to CED last week and asked them if they're newest model chrony could be used with a larger skyscreen spacing. They told me no, only designed for the 18" spacing. They said a company in Germany tested the unit and determined that there was no benefit to a larger spacing. The 18" spacing was providing just as accurate readings as longer spacings were.

Regardless, I agree with Lou Boyd. If conditions are perfect and everything is working properly, their test results and conclusions 'could' largely be true. But their hired independent test firm probably ran the tests with the IR lighting in use and eliminated all other common sources of influence deterimental to chronograph performance that could commonly occur in field use without IR lighting. Greater skyscreen spacing should help reduce the percent of error relative to lesser skyscreen spacings. Even though CED said their instrument can't be reset to calculate velocity at greater skyscreen spacing, I'm sure one could increase the spacing anyhow by 2x, 4x, or 6x, and then it would be necessary to increase recorded velocities by 2x, 4x, & 6X, respectively.

I've said it several times in other threads. Until you record your bullet velocity over two chronographs set up in series so that you receive two recorded velocities for each bullet fired, you really have no way of determining if you're getting good data or bad data. Your eyes will then be opened and you'll be able to identify good data when you see good agreement between the differences in the velocity recorded by the two chronos, and to disregard bad data when the delta in the recorded velocity is outside normal delta for your setup.

You still won't know which chrony is providing the most accurate, absolute velocity. But at least you'll be able to identify and use good recorded velocity data, consistent with delta in velocity from your units, and have a solid basis trusting those data. Which is enough to ensure that the data can be used to determine accurate ES, SD, and BCs (if you so choose to determine your bullet BC by two recorded velocities over a known range).

You collect chronograph data over one chronograph and you're shooting in the dark. Shoot over dual chronos in tandem, and you can identify and discard bad sets of velocity data. You still generally can't absolutely know or prove which chrono is acting up, but you can at least ID and disregard suspect (bad) recorded sets of velocity and by doing so, the data that you do use to reach conclusions can lead to valid conclusions. Ken Oehler certainly recognized this - thus as shooters became more demanding they marketed their Model 35 with the proof channel. If I'd known then what I know now, I'd have purchased one or two of those when they hit the stands.

 

[BryanLitz]

Very good insight Lou,
I agree with all your comments about chronographs. The clock is rarely the weak link in the units accuracy, but the sensors are where the questions come in. Stretching the spacing out is a sure way to reduce uncertainty in velocity measurement. I'm interested to hear what CED's comments are regarding the potential of longer screen spacing.
Jim Ristow (creator of RSI software and involved in CED design) told me that the CED sensors trigger off the base of the bullet rather than the tip because the base is better defined from an optics point of view.

Not to derail the topic, but has anyone had experience with the PVM chronograph? Anyone know where to read an objective, comparative review? They're real expensive (~$700 I think), but might be the answer to chrono uncertainty question.

-Bryan

 

[Jon A]

I've used an Oehler model 35 for many years. It came from the factory with a 2 foot spacing rod for the sensors. Thats 1 foot between sensors. After a few sessions I made an 8 foot spacer and that improved my handload's measured SDs noticably. That's not to say the Oehler is a bad unit, only that you can't make reliable velocity measurements with a 1 foot measurement base unless the sensors and their alignmet are near perfect.

Good stuff Lou, but I need to correct the above. The Primary measurement is taken over the 2' spacing with the first and last sensors. The middle sensor provides another reading for the Proof channel with only 1' spacing. The accuracy of the Primary measurement is twice that of the Proof measurement. Likewise, if you put them on a 4' or 8' rod, you're getting every bit of accuracy available, the middle sensor still only gives a doublecheck with 1/2 the accuracy.

 

[Mikecr]

I use 20ft primary screen spacing on a 35P, and found it to be the biggest ES improvement within my capabilities.
But it still does not accurately reflect MV.
For one, there is around 12-15fps of velocity decay over this range(20'), and a few fps between the muzzle and first screen, so averaged amounts to ~8fps low & unaccountd for.
You can also introduce more error if the bullet track isn't parallel to screen centerline. It don't take much..
I use a laser bore sighter to center the shots 5" above all sensors, and level them as well..