Chronograph accuracy?

[DRNewcomb]

I'll stick this in "Reloading" because that's where I associate chronographs.

I used to have access to an Oehler chronograph that had three sensors. If the comparison between sensors was off, the shot would be marked as an error. I no longer have access to that chrono, so I bought a nice little CED. I've only run about 50 shots through this new one. I'm really just getting used to it. In every string there's the one that makes me say. "Hmmmm, I wonder if that shot's velocity was that far off or if something's not right with my chronograph?" ( say one shot 30-50 fps off).

I'm curious if anyone has ever tried dual (dueling) chronographs and compared the readings? Maybe I have it a bit to close to the muzzle? I'd like to figure out if I have a chronograph (usage, setup) issue or if I need to work on my reloading technique?

 

[CatShooter]

I'll stick this in "Reloading" because that's where I associate chronographs.

I used to have access to an Oehler chronograph that had three sensors. If the comparison between sensors was off, the shot would be marked as an error. I no longer have access to that chrono, so I bought a nice little CED. I've only run about 50 shots through this new one. I'm really just getting used to it. In every string there's the one that makes me say. "Hmmmm, I wonder if that shot's velocity was that far off or if something's not right with my chronograph?" ( say one shot 30-50 fps off).

I'm curious if anyone has ever tried dual (dueling) chronographs and compared the readings? Maybe I have it a bit to close to the muzzle? I'd like to figure out if I have a chronograph (usage, setup) issue or if I need to work on my reloading technique?

I put three Chronos in a line. It was a BIG wake up event.

They were all off, and were not consistent in their errors.

One might expect an error between units, based on the quartz clock error... but at least the error should be constant, i.e., unit "one" should always be 0.0XX% faster (or slower) than unit "two"... makes sense.

It doesn't happen - they were all over the place... one would be higher on one shot and lower on the next.

All three were high end units with excellent reputations - all three had the shield supports painted black... and it was an overcast day, so there were no "glint" issues.

I spoke with Ken Oehler about it, and the conversation lasted 45 minutes.

The summery was that chronos give you approximations of velocity, not exact velocities, and there is no practical way to check the accuracy.

Much of the errors are due to the design of the traps, and the fact that better traps are more complicated than 99% of the shooters would be willing to pay for and be willing to fool with.

 

[brian427cobra]

I use a CED M2, I found mine to be pretty accurate, now, it wasn't until I started using a Prometheus Gen1 and now a Gen2 that the AVG SD/ES numbers fell in line with what my targets were saying at 1000, 1350 and 1780. My advice is dont discount what it's saying until you verify at distance measuring real time conditions with a Kestral and a ballistic program running.

 

[DRNewcomb]

The summery was that chronos give you approximations of velocity, not exact velocities, and there is no practical way to check the accuracy.

Much of the errors are due to the design of the traps, and the fact that better traps are more complicated than 99% of the shooters would be willing to pay for and be willing to fool with.

Thanks! Very educational comment.
Are the "traps" part of the sensors or part of the electronics (e.g. A/D converter).
If one of my buds gets one of those MagnetoSpeed chronos I'd like to try an inline comparison some time.
Is there a variability that you can just expect to happen with most hobbyist-level chronographs. (i.e. Don't really worry about 50 fps, just average it.)

BTW, I will say that in my two chronographing sessions with this CED I really shouldn't have expected low ES & SDs. In the first session I was loading a powder too slow for the cartridge and was burning half of it somewhere out beyond the muzzle. In the second session I was working up loads for some canister-lot powders I bought from Pat McDonald 20 years ago and had just weighed the powder into some new LC-77 cases and seated some old Hornaday 168s without even trimming or neck sizing the cases.

 

[CatShooter]

The summery was that chronos give you approximations of velocity, not exact velocities, and there is no practical way to check the accuracy.

Much of the errors are due to the design of the traps, and the fact that better traps are more complicated than 99% of the shooters would be willing to pay for and be willing to fool with.

Thanks! Very educational comment.
Are the "traps" part of the sensors or part of the electronics (e.g. A/D converter).
If one of my buds gets one of those MagnetoSpeed chronos I'd like to try an inline comparison some time.
Is there a variability that you can just expect to happen with most hobbyist-level chronographs. (i.e. Don't really worry about 50 fps, just average it.)

BTW, I will say that in my two chronographing sessions with this CED I really shouldn't have expected low ES & SDs. In the first session I was loading a powder too slow for the cartridge and was burning half of it somewhere out beyond the muzzle. In the second session I was working up loads for some canister-lot powders I bought from Pat McDonald 20 years ago and had just weighed the powder into some new LC-77 cases and seated some old Hornaday 168s without even trimming or neck sizing the cases.

The "traps" are all the parts used to gather the information, up to the inputs.

To call them "Micky Mouse" would be an insult to The Mouse.

 

[DRNewcomb]

The "traps" are all the parts used to gather the information, up to the inputs.

To call them "Micky Mouse" would be an insult to The Mouse.

Sky-screens, lens, optical sensors, wires, etc?

 

[CatShooter]

The "traps" are all the parts used to gather the information, up to the inputs.

To call them "Micky Mouse" would be an insult to The Mouse.

Sky-screens, lens, optical sensors, wires, etc?

Yes - the wires are OK the rest is shakie at best.

 

[dave7mm]

I have a Oehler Model 33 I bought in 1986.
3 years ago I used it to measure by best load out of my then new 300 RSAUM 17 pound 1000 yard rifle.
I used the data I got from a 10 shot string and plugged it into the free Berger program.
After taking some careful measurements I dialed the come ups into the S&B PMII and made first round hits at 850,900,950 and 1000 yards.
If i missed it was because of the wind not the dope I dial in.
Only thing I do different then most people is a screen spacing of 10 feet instead of 5.

dave

 

[RonAKA]

I have a Shooting Chrony Alpha model, and was having some repeatability issues with it. I had access to another identical chrony and set them up in series in my usual way. There were quite wide differences in the velocity measured by each of them.

I then did a second test where I moved both of them as far out as the cord would reach (further than I had been setting them up previously), and used a level to accurately level each one in both directions. I then used the narrow setting of the angular guide rods, and only one rod on each side with no extension. The second test gave velocities out of each chrony that were nearly identical to each other, with the % error insignificant.

I don't bother to use two any more, but I do take the extra time with a level to set the chrony up perfectly level and as far away from the muzzle as is possible with the standard cord. Muzzle blast and misalignment was clearly affecting accuracy.

 

[DRNewcomb]

The "setup" of a chronograph is very important, and is a largely over looked aspect. Fallow ups with a consistent setup and spacing ever time, goes far to repeatability.

I think this is where I may have a problem. I spoke with some of the guys at the F-class match yesterday and one of them said that he makes the point of always measuring the distance from the shooting bench to the first screen to be sure it's 10'. I've just been saying, "That looks 'bout right." I also suspect that the photographic tripod I use to set up the rail & screens may be a bit too wobbly. I was having some issues with the sensors being moved around by the wind and muzzle blast. Unfortunately, at the club where I'm shooting, the rifle range is busy enough that I can't always take as much time as I need to tinker with the setup.

 

[amlevin]

Lots of things have to happen while the bullet's traveling a short distance.

First, assuming that the "head" is working perfectly fine, the sensors "right on", then how does one know what part of that inch long bullet is actually triggering the sensor?

On a chronograph with an 18" mounting bar/rail for the optical units, there can be at least a 2% error with just a 1/2" variation of "trigger points. If one is triggered by the point of the bullet and the next by the larger body from ogive back, there goes your uniformity.

Toss in electronics and it gets even more fun.

I have been accumulating some chronograph parts. I have three "Heads" for PACT chrono's. Shooting ammo across one setup shows nice consistent single digit spreads in velocity. Merely swapping the head, leaving the sensors in place with the same lighting conditions, can yield not just up to 100 fps average speeds but also show wide spreads.

As one of the more accomplished shooters at our range stated, just use the chronograph that works reliably and then evaluate your results on the target.

That is unless you're an Engineering type and want to approach this like a "Moon Shot".