CED M2 Chronograph

[Doc Hoy]

Guys,

Permit me once again to sing the praises of this instrument. I know that plenty of folks have good success using other chronographs. I was unsuccessful with two "Shooting Chronies". I was just about to give up when I got a recommendation for this one.

I get good consistent speeds. The range of speeds gives me good confidence that the device is accurate. I love the fact that the display/comtroller is on the bench and the traps are fifteen feet downrange.

I am disappointed in the software that comes with the machine since all it does is display the data input from the chronograph. I am looking for ways to export the data to Excel but to this point all I am doing is loading the data manually. I have a spread sheet hooked up to present 1st standard deviation, Max speed, Min speed and average.

This thing is going with me every time.

[mykeal]

Doc - did you get calibration data with the unit? Does the device come with instructions on how to calibrate it? How do you know it's not simply reporting the same errors over and over again? Getting the same numbers repeatedly does not indicate accuracy.

[[email protected]]

I'd like to have one but I probably would hit the trap my groups are so darn wide.

Who made yours Doc or is that the company name you showed?

[Doc Hoy]

It has the same calibration instructions and certification that comes with the Shooting Chrony.....None

Apart from the manufacturers caution to make sure it is set up properly to preserve accuracy and performance. I thought I read accuracy data when I was buying the thing but as I look through all of the websites, I am not able to find anything.

[Doc Hoy]

The CED stands for Competitive Edge Dynamics.

[bedbugbilly]

I have what is probably a "stupid question" but then what's new about that? I admire all of you guys who are scientific about using a chrono, checking fps, and all of those things - without your efforts and info, improvements, etc. wouldn't happen. So . . . I'm not being a smart A _ _ when I simply state that I've just never worried about things like that - I shoot to shoot - to "throw lead" and see how close I can come to the bullseye, tin can or if I can possibly ring the "gong". I've just never been concerned about such things but I do admire and appreciate those of you that are.

I'm just curios . . . . I've never given it much thought . . . just when did the scientific study of such things as bullet speed, etc. come into existence? I'm sure that it has always been there, but, what DID they do back "in the day" - say the 1860s to check on such things? How was it tested? I'm sure that the computer age has made it much easier and with the advances in technology, the price has come down where most shooters could own and use a chrono if they want to. As I said, I'm not of a "scientific mind", but I am assuming that the use of such a device helps you to adjust your load, especially in a cartridge, to impove your target score and given distances. Everyone is interested in improving such things. I have done a lot of roundball rifle shooting and of course, I would adjust and play with the number of grains of powder to improve on my ability to hit the target - but - I also have always depended a lot on really "knowing" my rifle and using "Kentucky windage" to compensate for certain conditions.

Doc - I know that you do a lot of shooting and as a part of that, using your chrono is an important part of it. A sort of "which came first, the egg or the chicken" question . . . do you (or others) use the chrono because you are attempting to "zero in" your target hits . . . or . . . do you use your chrono because you are attempting to work up a load that meets a certain fps? I've never had anyone explain it to me and I'm just curious. Maybe you could explain why it is important to use one? I've never understood physics that well and I am wondering after reading a number of posts on the chrono - is there a reason to achieve a certain fps? I mean . . . if you have a load that allows you to hit a target at a given distance, does it matter whether the bullet is traveling at say . . . 900 fps instead of 750 fps? In my simplistic way of looking at it, I'm assuming that the higher fps the projectile travels, the greater penetration on the target or "kill advantage" as opposed to a slower moving projectile. I'm also assuming that the higher the speed, the flatter the trajectory of the projectile. What I'm having trouble wrapping my mind around is if you are shooting targets at a specific range and you know how your weapon shoots with a certain load (say 22 grains of FFG), regardless of the fps, it should be fairly consistant with the same load each time (ruling out fouling, etc.). Idealy, when you learn how your pistol shoots, you should be fairly consistant with your hits - regardless of the fps, the major factor is that the muzzle is pointed at the right spot for how that load shoots.

I'm hoing that you can shed some light on some of my questions and/or comments and feel free to "shoot holes" in my assumptions. Again, I'm not poking the bear for those that get scientific with the chrono studies, etc. - I'd just like to understand the use of one better and the whats and whys of them. I don't know if I would ever use one very much but maybe there is something I have been missing out on by not having one. Many thanks for any info or explaining that you can do on it - it is greatly appreciated!

Sincerely,
Bedbug

[NWPilgrim]

I too have the CED M2 chrony but have only used it a couple of times. One thing I learned is that if you live/shoot in a consistently sunny or overcast place you don't need the infrared sky screens. But, if like me you live in a variable climate (NW Oregon) you really want to have the IR screens. My first session was spent constantly taking the normal screen off and on as the sun was bright and then hidden behind clouds. I barely got a few complete velocity strings between all the changes back and forth.

As the sun lowered in the sky then I was not able to get any readings.

As far as I know about chronies this is not a trait of CED specifically but a limitation on chronies in general. The IR screens however give a constant level of IR light to contrast the bullet passing and therefore can be used in all light situations including rapidly changing light.

As Doc says you get no real calculations built into the software just the raw data, but Excel and many other programs will do the calculations using the raw data.

I'm very happy with the M2 along with the IR screens.

[Doc Hoy]

Hope this is not too long to read.

I think your questions are very valid.

In point of fact the measurements I take with this chrono may prove to be of very little value. I do it because I enjoy taking and analyzing the data. If I were ever to prove anything or to develop a revision to the way I shoot, I would be doing backflips.

As regards early tests, I am sure this is not the first but I read of a test performed by the British army during the unpleasantness with Napoleon. A battalion of soldiers armed with smooth bore muskets fired at a panel of cloth which was roughly the size and shape of the frontage of an opposing infantry formation drawn up in line. A single volley was fired at differing ranges. The holes in the cloth were then counted as a way of predicting the likelihood of the infantryman scoring a hit on an enemy combatant.

It was learned that at 200 yards, the formation could stand with impunity and without fear.

As regards the question on my purpose in using the chronograph I would say I do it because most of the data produced is interesting to me, not because I would hope to improve something but more as a way of knowing something. I get enjoyment out of collecting and analyzing the data. If I really wanted to improve something, I would begin elsewhere. For one thing I would shoot far more often than I presently do.

On thing that I have always worked toward is the reduction of variance. I started with the consistency of my round balls in size and weight. I think I have made real progress in that area. This most recent test using the chronograph seems to imply that the consistency of the ball has a lot to do with consistent MV. I would have to take a lot more measurements adding some additional controls to be able to say that I have confirmed that. But I do agree with Mykeal and others that MV may have nothing to do with accuracy per se. I also agree that accuracy is most important.

You asked if I use the chrono to work up a load and the answer there is, "No" because I have not got to that point yet, although I would say that most of the serious shooters who use a chrono are using it for that goal. I am not that sophistocated.

I am hoping to learn that as I am able to reduce the variance in the various aspects of my shooting that accuracy of the weapon will get better. Accuracy is a reduction in variance too. I am not saying that I think that two bullets of the same size are more accurate than two bullets of different sizes. Only that two bullets of the same size are more likely to behave the same than two bullets of different sizes.

I think you are right about shooting at a target as a means of reducing that accuracy variance. I also think most people approach it in that way. In fact, up until about three weeks ago, that is the way I approached it. Now I have this chronograph and so I am just getting accustomed to it. You can well imagine how excited I was when I saw the correlation between ball size consistency and MV consistency. I had already reduced the variance in the powder charge because I started using the measure contraption that I came up with a coupla weeks ago. Variance in my powder loads is very low and FWIW, I am quite proud of that measure even though many shooters think it is dangerous to load a previously fired revolver from a flask.

[NWPilgrim]

My intended use of the chrono is for comparative purposes, whether it is accurate to an absolute velocity reference is not important for me. At any rate, short of shooting under laboratory conditions (special test barrels, temperature and humiduty control, etc.) I am not sure you could ever get absolute "accurate" velocities. Just too many variables from some testing lab to my range conditions.

At any rate what I am looking for is how my loads compare to a common factory load I use. My local cheap ammo is Remington UMC. So I would like to know how fast or slow my pistol or rifle ammo of the same weight compares to UMC.

Secondly I want to know how well one load using powder A compares to another of my loads using powder B. For instance, If I am loading 75gr OTM bullets in .223 the velocity is dropping pretty quickly rom the normal 3,200 for a 55 gr. bullet. Once I find an accurate load for this bullet with each of a few powders such as Varget, BL-C(2), TAC and RL15 then I would like to know which powder gives me the best combination of accuracy and velocity for longer ranges.

Thirdly, consumer level chronies can still provide some measure of safety. If the load book shows a powder at max should give 2,800 fps velocity out of a 22" barrel and if one of my test loads starts hitting near that even before I have reached powder weight then I would take that as a warning that either I made a mistake or my rifle/ammo combination is running hotter than the loading manual conditions and I should not shoot any loads with more powder than what gave me the book max velocities. The chrony maybe off 50fps or 100 fps, who knows for sure, but I doubt they are off more than that from absolute and once I get ammo into that neighborhood I am calling it good enough. This would be more critical for someone purposely loading beyond the listed powder weights for experiments, but I am just a recreational loader and shooter. It's like your car speedometer. Does anyone have a truly accurate speedometer, or are most within an acceptable accuracy of +/- 3mp so who cares?

Fourth, like Doc I am just naturally curious. After handloading for 20+ years I kind of want to know what some of my best loads are clocking.

[mykeal]

serious shooters who use a chrono are using it for (working up a load)

If they are, they're fooling themselves. Chronographs measure velocity, not accuracy. Working up a load means developing the most accurate load, not the fastest or slowest, or any other 'est' with respect to velocity.

what I am looking for is how my loads compare to a common factory load I use

The chronograph will tell you how the velocities compare, but nothing else. If that's all you want to know, fine, but if you're implying that the chronograph, or the velocity comparison, provides a performance comparison between your loads and a factory load, I don't believe you're getting what you paid for.

I want to know how well one load using powder A compares to another of my loads using powder B

Same as above. Velocity only, not performance.

chronies can still provide some measure of safety... as a warning that either I made a mistake or my rifle/ammo combination is running hotter than the loading manual conditions

Or that the chronograph is not measuring the data the same as the one the factory used for the published data. Frankly, without comparative calibration data, which nobody publishes, you have absolutely no idea how your data compares to theirs.

The chrony maybe off 50fps or 100 fps, who knows for sure, but I doubt they are off more than that from absolute

Errors are stated in percentages, so if you're measuring 1,000 fps and your chronograph is of 50 fps, the error is 5%. That's pretty poor for good instrumentation, but probably not too far off from what some of the consumer level chronographs produce. 100 fps, or 10%, would be flat out unsatisfactory. You make a good point, however: do we really care?

[bedbugbilly]

Thanks fellas - you've cleared up a number of questions that I hae been wondering about in regards to the chronograph. I can see all of your points and I greatly appreciate your replies to the questions I had - thanks a million! Sincerely, Bedbug

[arcticap]

I think that consistent velocity does lead to better performance and accuracy.
That's because just like with all good ammo, in order for consistent accuracy one must find the most consistent load that shoots the most accurately.

How does one know if a good group is a fluke?
To obtain repeated velocity measurements of the same loading and then measure groups with that load over and over again.

So I think that accuracy is the result of consistent velocity in conjunction with the right size ball, barrel twist rate and loading methods etc...

The velocity isn't meant to be measured in a vacuum, but is an important tool in a person's "accuracy arsenal".

I think that match target shooters would have more to gain by studying their chrony data.
It's also a way to test powder performance from lot to lot.

And in that way more consistency and scientific method can be introduced into the shooting sports.

I just like to sling lead downrange myself, but that doesn't mean that we all can't appreciate the knowledge about how the science of ballistics applies to everyday shooting.
If chrony knowledge promotes a better understanding about how our guns perform, then that's a positive benefit and not a negative one.
Even if the chrony data was only relative and not absolute, then it would still provide for comparative analysis.
Chrony programming and results are better than nothing and are actually meaningful to each and every individual gun tested and not just the writer's sample of a published article.
We live in the space age, people demand performance from their guns and all of the products that they spend their hard earned money on.
If an expensive mold didn't throw a good round ball then how are we suppose to easily measure it without some modern electronic calipers for instance?
Or if we want our percussion caps to fit the nipples of our new guns, don't we want to be able to measure the nipples and don't we expect the manufacturers to provide caps and powders that live up to certain manufacturing standards?
How consistent does Diamondback powder shoot?
I guess that what I'm getting at is that scientific instruments are affordable enough for everyone to be able to obtain some benefits from them, and not just the companies and the book publishers that can afford them.
It's a new electronic world, and manufacturers are able to build to more precise tolerances and standards then ever before. And how the average shooter can test performance of their products is by using instruments like a chrony and then analyzing the data.
How else would we know if Goex was consistent or if 777 powder was 15% faster or not?
Is Swiss powder worth the money?
Does each and every chamber of a new revolver produce the same velocity and accuracy?
A chrony can help to answer some of these questions and many more for those so inclined to investigate.

[NWPilgrim]

Bedbug, you are reading too much of your own judgement into my post. I said nothing about using the chrony for accuracy. that is your pet peeve, and has nothing to do with why I am using it.

I also never said I was trying to see if my chrono readings would tell me how my ammo would 'perform", that is your wording not mine. I am only referenceing comparative velocities. To be 100% accurate a chrony only measures speed since velocity is a combination of speed and a directional vector and I don't know of a chronograph that measures absolute direction of bullet travel, only that it passes through the sky screen window.

There are plenty of interesting things for which a consumer grade chrony can be used. If you don't want to use one fine, but don't **** on everyone else that is interested in what they can find. I managed to build good handloads for 20 years without a chronograph but now that I have most other equipment I need/want I find the added info from a chrony to be useful and interesting. I also managed to hike for 40 years without a GPS but now that they are affordable I find it very handy.

I think we all agree a consumer grade electronic device will not give absolute perfect velocity readings. My home thermostat is not 100% accurate, neither is my vehicle speedometer, my GPS, my watch or anything else. Yet somehow I muddle through life with reasonable success.

[mykeal]

NWPilgrim -

I think you need to apologize to begbugbilly. I believe the post you were responding to was mine, not his.

[Doc Hoy]

Which is not surprising.

Let me paraphrase and while I am doing it, hope that I am not putting words in Mykeal's mouth. If I misspeak, Mykeal, please correct me and accept my apology.

I took from his early response that Mykeal holds that MV is at the most, a small contributer to accuracy. A point with which I think most of us would agree. We all know about projectiles (ballistic projectiles) traveling at speeds from 300 fps to over 3000 fps which are equally accurate in practice.

I think I understood from his post that the quest for accuracy is best undertaken by considering all impacting factors and then making a wise decision about which ones to focus on up front.

In my own defense I wish to make it clear that in taking chronograph measurements, I am doing so, less to serve accuracy, and more to salve my ego about my bullet casting. In my own mind, right or wrong, I think I have observed a behavior (one single small and very likely insignificant behavior) of the round balls I cast in comparison to the round balls I bought. This observation suggests that round balls chosen at random from a population of balls that I cast seem to be accelerated more consistently than round balls chosen at random from a population of balls that I bought. In fact, that would be the hypothesis, one which has yet to be proven.

Every researcher knows that one of the hardest things in scientific observation is reducing to zero, the impact of confounding variables. To the best of my ability I did that but I admit that far more scientific observations have been made than those I made in Gary Jones' back yard with my 200 dollar chronograph, my home made powder measure, my kitchen recipe bore lube, and the pistols which when I am not shooting get stored in my bottom drawer.

As regards the accuracy of the chronograph, I am quite familiar with the design of the device and the operation which results in the display of bullet speed. Producing a chronograph with extremely high INTERNAL accuracy is a very easy thing to do. Taking any chronograph in existence and ensuring that a specfic bullet speed has been accurately displayed is quite a different matter. As Mykeal said, the measurement is subjected to too many confounding variables.

In my original post, my function was to "sing the praises" of this CED M2. It has worked better than the two Shooting Chronies I had in three areas:

1. I get almost zero "error reports" from the device. That is not an indication that it is accurate, only that is works more often than the Shooting Chronies which were almost unusable.

2. It puts the display on the shooting bench rather than downrange with the traps. (One of my Shooting Chronies had a remote display but the device was nowhere near as user friendly as the CED)

3. The operating features of the device make it easier to understand the data it is presenting (right or wrong).

[mykeal]

Doc,

You restated my thoughts accurately; thank you.

Just a short dissertation on chronograph operation in general, in response to another member's assertion:

Chronographs actually measure only time; they then calculate and display velocity. And yes, it is the vector quantity velocity and not the scalar quantity speed. The vector that's calculated may or may not be a component of the projectile's vector - that depends on whether or not the projectile is traveling a path parallel to the chronograph's axis of measurement. If it's not, the calculated vector is simply a component of the projectile's true velocity and thus one of the many possible errors.

Doc, you are absolutely correct that the internal calculation of velocity from the empirically derived time value is extremely accurate in even the least sophisticated chronographs.

I don't have any experience in actually setting up a chronograph, but how well it's measurement axis is mounted with respect to the projectile's velocity vector will have a great deal to do with how accurate the results are. Is that easy to do? Just putting the bullet through the wire loops isn't enough - you need to be sure the axis is properly aligned. This may or may not be an issue - I don't know.

And one final observation. On another forum, dedicated to traditional muzzleloaders (which means they eschew in-lines and cartridge bp shooting) there is a large (over 10,000) group of well seasoned practitioners who are fans of the Dutch Schoultz's Black Powder Accuracy System. I've used it myself and will guarantee you it results in simply stunning results: 50 yard 3/4" 5 shot groups repeatedly and consistently. It's essentially a method of working up the best load (ball, patch, powder and lube) that works in your rifle. It costs $20, and you have to burn a lot of powder and spend a lot of time preparing for each shooting session, but the results speak for themselves. And nowhere in the system is a chronograph even mentioned.

[NWPilgrim]

Thanks for the correction, Mykeal. My apologies to Bedbug who was ASKING questions.

The operation of the chronograph is very simple. It measures the time difference between a contrasting object passing between two or three sky screens set at a known distance from each other. From this it calculates the speed of the object.

First the object has to make a contrasting presence at each skyscreen. If you rely on ambient light then it has to be bright enough for the sensors to distinguish the background and the object contrast. Thus indoor light usually is not enough. Same goes for a setting or rising sun low on the horizon. An overhead sun will give best contrasting condition. Or an artificial light source (infra red diodes) and corresponding sensors which make the screens independent of ambient light conditions.

The object has to be small enough that it can cleanly pass one screen with a measurable interval before it passes the next screen. Thus a bullet can be sensed separately but an arrow will be in both screens at the same time and the device will not be able to measure a time difference. So length of the object can be a factor if it becomes a significant percentage of the distance between screens.

The speed calculation is based on the difference in time an object appears between a known distance. So it is important to assemble the screens properly and make sure they are in their absolute furthest settings on the rail. If they are tightened down a bit short of the max distance then the calculations will not be correct by that same percentage.

[mykeal]

No comment on the error due to a misaligned flight path or misaligned sensors?

If it's important that the screens be set to maximize the distance between them, it follows that the velocity calculation assumes that distance is the one to use. If the projectile takes a different, longer path, for instance, a diagonal between the screens (either horizontally or vertically or both), then the calculation will be in error by the amount the actual path distance varies from the assumed path distance.

That error can be characterized; what are the dimensions of the screens and their locations? Do the screens have a method of aligning the theoretical path with the intended actual path?

[Doc Hoy]

Chronographs measure time....

This is why they are called "chronographs" and not speedometers or velocimeters or How-fast-was-that-bullet-meters.

The manufacturers would have you believe that the clock speed is a primary parameter in chronographs, but in fact, if the clock speed is reduced to a lower frequency to sample the elements in the trap, then clock speed is not important.

The clock speed of the CED is 48 Mhz the traps are 24 inches apart. A projectile entering the trap at 1000 fps is in the trap for one five hundredth of a second or 2 milliseconds. In that time an accumulator will store 96000 pulses at 48 Mhz. (I assume the clock frequency is converted to digital which is why I call it "pulses.") The number of pulses in the accumulator is converted to a bullet speed. More pulses means lower speed so it is not a simple division by 96. More likely the accumulator is preloaded with a reference number before the shot and than the number of pulses counted as the bullet travels through the trap is subtracted from that reference.

It is an extremely simple process which was actually developed at about the same time science gave us photography.

[arcticap]

Originally Posted by Doc Hoy

I took from his early response that Mykeal holds that MV is at the most, a small contributer to accuracy. A point with which I think most of us would agree. We all know about projectiles (ballistic projectiles) traveling at speeds from 300 fps to over 3000 fps which are equally accurate in practice.

I do disagree that velocity is only a small contributor to accuracy. And it's generally well established knowledge that projectiles @ 300 fps aren't going to have the same degree of accuracy as projectiles @ 3000 fps.

There are barrel harmonics, barrel twist rates, the characterisics of the projectile and powder among other variables. But velocity is as important as any of those. Just ask any Olympic rimfire shooter what the difference is between one type of ammo from another, or one lot from another, even if it's made by the same company using the same bullets like Eley.
The difference is in velocity and type of powder, even if it only affects accuracy to a very small degree where one point is the difference between earning a gold medal or not. Maybe that's not as noticeable with muzzle loading pistols and rifles, but consistant velocity is very important to match rimfire shooters around the world.
Why else would someone be concerned about measuring their powder charges very precisely if it wasn't for its potential effect on velocity, and in turn accuracy?
Velocity is so very important because barrel harmonics greatly affects accuracy. It's been proven. Just look at the Boss system and how a barrel tuner can help to dial in ammo of a certain bullet shape and velocity. Barrel harmonics are vibrations that produce a frequency of barrel whip that occurs during each shot. Consistent muzzle velocity helps to regulate the barrel whip (harmonics) so that the bullet nearly travels in the same exact flight path each time as it exits during the same point of movement during the barrel whip. An ideal constant velocity then becomes much more critical for any given bullet design shot from a particular gun.
That especially holds true since most guns don't have harmonic tuning devices attached to their barrels. So most shooters can only control the bullet style and velocity of it exiting the barrel in their quest for optimum accuracy from their loads.

[mykeal]

I think you're confusing consistency with velocity, if that makes any sense. You make a good case for 'consistent velocity' as being important to accuracy, and I can agree with that in the sense that if you aren't consistent, you don't know where the bullet is going, and that's the very definition of lack of accuracy.

But is any one velocity, 1250 fps for instance, more accurate for a given combination of bullet, rifle, powder, lube and patch than say 1300 fps? I haven't seen any evidence to support that assertion. But maybe I don't understand your argument; maybe that's not what you mean when you say velocity is important to accuracy.

Or put another way: If I manage to obtain, say, a 5 shot 3/4" group at 100 yards with a given combination of variables, and then determine that all 5 rounds were measured at, say 1500 fps, and then let's say that I reduce powder and projectile weight to get another combination of variables that measure 1500 fps, consistently, does it follow that the second combination will necessarily produce 5 shot 3/4" groups as well? There may well be a second combination of variables that produce that accuracy, but I submit the velocity won't be the same. Or will it?

Velocity can be an indication of how closely the components of a given round matched those of another round, but in my opinion that's all it is: a metric. Perhaps we're actually in violent agreement.

[arcticap]

I'm not confused about barrel harmonics and how velocity affects it. Consistent velocity equals repeatable accuracy, but the barrel hamonics must also be tested for each bullet type/weight/length fired by reading the target results so that the optimum velocity range can be found. It's basically working up a load using a chrony and introducing velocity into the accuracy equation. Velocity is usually simply measured by the amount of powder loaded, but the FPS isn't known for when switching powders and neither are the deviations known. They are only guessed at without knowing about the amount of human error. Using a chrony is just a scientific and provable method of determining what the velocity is without simply relying on the amount of powder loaded.

[mykeal]

arcticap - apologies. I edited my latest post to add another paragraph while you were posting your latest. It appears from the sequence that your latest responded to my latest, but readers should know I changed mine.

[arcticap]

I edited for several minutes too.

I agree that consistent velocity can produce consistent inaccuracy. But that's why I mentioned above that velocity isn't measured in a vacuum. It's used as the reference when shooting groups at targets to determine what speed the accurate or inaccurate load was traveling at. It's simply a reference point so that there's a standard for when switching powders, or different lots of powders to reformulate the next optimum load, especially for competition shooters.

When trying to configure a new load or match a successful load, one of the things that can almost be guaranteed to be achievable is the velocity. Then at least there is a basis to tinker with a load to try to successfully match the velocity and accuracy of a previously accurate load, even if the powder has changed.

[NWPilgrim]

If the projectile takes a different, longer path, for instance, a diagonal between the screens (either horizontally or vertically or both), then the calculation will be in error by the amount the actual path distance varies from the assumed path distance.

That is the theory. In practice the difference is insignificant. I find the sensors work best when the bullet is near the bottom of the "V" of the skyscreens. I set them up to be shooting about 6 inches above the sensor apex (bottom of V).

You normally align the screen rail fairly parallel to the direction of the target as you do not want to be hitting one of the screen arms.

I would estimate at most you could be 3" off horizontally from one screen to the other. But when you calculate the difference in length it is only 0.77% more going at that diagonal. At 3,000 fps that would read about about 22-23 fps less than if it was traveling through both centers exactly. For me, 23fps is insignificant for my purposes.

Since I am intending to compare loads during the same shooting session then I would estimate my shooting through the screens only varies by about 1" horizontally, if that much, during the same session. That reduces variance from diagonal paths to about 7fps.

It is a mistake to try to be too precise with a consumer grade electronic instrument under field conditions. There are many other variables that have much more effect on variance than the chronograph: temperature, handloading technique, powder measure tolerances, etc.

For instance most popular powder measures have about a 0.1 gr variance. With a light handgun target load of 4.0 grains that is a 2.5% variance alone. Or, many rifle powders have been shown to vary as much as 100fps (at around 2,800 fps nominal) due to wide temperature changes (Hodgdon Extreme line of powders is designed specifically to minimize temperature variation in burn rate).

Like any tool you have to understand what a field chronograph is designed for an use it within those parameters. They are used widely in the shooting community and I have never heard of any brand being known for significant errors. My experience with the CED M2 is it either gets a good reading or it tells you it can't determine the speed at all. Some chronies have three screens so it takes two or three sets of readings and can determine if one of them is out of line.

I bought a boatload of reloading equipment before I considered getting a chrony. They certainly are not essential, but they do you give you another way to measure your ammo characteristics and compare them among others you measure under similar conditions.

Just a comment on the speed and accuracy discussion. Speed itself has nothing to do with accuracy, but consistency of speed usually does at some point. However, when a bullet drops below the sound barrier it can introduce greater variability. So a chrono can help you determine whether your bullet will remain supersonic or maybe dropping below the transition range and more likely to be less accurate.

Chronos are also used at some matches to determine the "power factor" of each participants loads. So it is handy to have one to make sure your loads will pass the testing before you show up on the day of competition.

If one is more interested in lab work than the shooting range then these are not the instruments for you and you should pan to spend thousands and not a couple of hundred dollars.

Note: I have purposely not identified every possible use of a chronograph in the field of recreational, nor every possible factor that may cause variance. I have mentioned the ones I find interesting or notable. I assume others can point any others that are relevant to them.