Large discrepancy between Chrono speeds

Gear Head

Gear Head

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So I ran a Chrono ladder today a borrowed magnetospeed and my Caldwell optical Chrono set up as well. I expected the magnetospeed to give me numbers a few fps faster than the caldwell since it was literally on the muzzle and the caldwell was about 12' downrange but this is what I got. Any ideas? Caldwell was set up in the shade so the lighting didn't change. If they we're close then it wouldn't be a big deal but with these differences I don't know which one is actually correct. Judging by loads I've found online it looks like the Caldwell data is closer to true and all the magnetospeed readings are slow.

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I have not run the numbers yet, but to me this would not be completely unexpected. When comparing two pieces of equipment that work on different principles to measure the same event, I would not be surprised to see a difference of 5% in the correlation between them. I would base my belief in them the correlation more upon the standard deviation of the two data sets. As to which equipment to believe, well...​
 
OK, so the difference between the two chronos is averaging right at 3%, with a range of 2.5%. I don't think that is too bad. What is more concerning is that the muzzle velocities do not consistently increase as the powder charge does, but where you see that anomoly, the chronographs follow the same pattern in one case and have outlier differences in velocity in another, so you are probably looking at a combination of loading variability and variability between the chronographs, the optical chrono having greater potential variability than the magnetospeed.

To determine which is correct, reload at one of the charge weights, load 5 of the same charge and average the muzzle velocities. Enter them into a balistic calculator for 200 or 300 yards and see which is more correct.

Now you are beginning to have some fun aren't you?

Me, I have found wearing a tin-foil hat when I load precision ammo helps.
 
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Not saying I'm a pro reloader but I took my time and trickled up to every charge and was confident everything was correct. It does follow the trend of slight increases mostly. The theory behind this load development method was to find a velocity "flat spot" where slight variances in charge weight don't result in much of a pressure change. So looking at the data 40.6-41.0 looks like a good spot on the caldwell but not so much on the magnetospeed.
 
" Last year TFB did an AMA (Ask Me Anything) with ballistic researchers Bryan Litz & Nick Vitalbo of Applied Ballistics. In response to a question about the MagnetoSpeed, Brian wrote:

The new book [Modern Advancements for Long Range Shooting] has a comprehensive chronograph review in which groups are fired over numerous chronos and results compared. In short, the magneto speed is very accurate and precise. I don’t hesitate to use it for serious testing in locations which prevent set up of the larger optical (Oehler) units. We did see some sensitivity to placement; you want it as close to the bore line as possible without damaging it.

… and Nick wrote:

The Magnetospeed is a good device. I prefer something like that over most optical measurement devices. With any type of variable light source, you end up with variable readings. Of course, the high end ones are pretty good and if you’re well acquainted with these devices and how to optimize results, then can definitely get good results.

The Magnetospeed is a good device – of course you have to watch placement. I am personally excited about some of the new RADAR devices as well as the acoustic ones as they could be useful."
 
ncav8tor said:
" Last year TFB did an AMA (Ask Me Anything) with ballistic researchers Bryan Litz & Nick Vitalbo of Applied Ballistics. In response to a question about the MagnetoSpeed, Brian wrote:

The new book [Modern Advancements for Long Range Shooting] has a comprehensive chronograph review in which groups are fired over numerous chronos and results compared. In short, the magneto speed is very accurate and precise. I don’t hesitate to use it for serious testing in locations which prevent set up of the larger optical (Oehler) units. We did see some sensitivity to placement; you want it as close to the bore line as possible without damaging it.

… and Nick wrote:

The Magnetospeed is a good device. I prefer something like that over most optical measurement devices. With any type of variable light source, you end up with variable readings. Of course, the high end ones are pretty good and if you’re well acquainted with these devices and how to optimize results, then can definitely get good results.

The Magnetospeed is a good device – of course you have to watch placement. I am personally excited about some of the new RADAR devices as well as the acoustic ones as they could be useful."
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So I was using the sporter which comes with a big and small shim. I was using the small shim and was 3/8" off the centerline of the bore which was the max distance recommended on the paper in the box. I was unsure if I was supposed to run it with no shim at all but that would get me about 1/4" close which would leave me about 1/8" off the centerline. Next time I get it out I'll try to get it closer and see if that makes a difference.
 
Gear Head said:
Not saying I'm a pro reloader but I took my time and trickled up to every charge and was confident everything was correct.
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Powder measurement is only part of it. There are many variables that can make the small differences seen in your data (for the most part less than 10fps). neck tension, even the temperature of your barrel and then you couple that with shooting through the same spot in your chronograph and shooting trough it straight, it all adds up. It takes a lot of perfecting to get muzzle velocities down into the low teens or single digits in variation.

Like I say, I think your data is good and useful for your purposes. I don't think any particular problems are indicated in it.
 
I watched some Johnny's reloading bench vids on youtube and he was getting very similar numbers to my magnetospeed readings using h4350 and 143 eldx so I'm going with the magnetospeed numbers moving forward.
 
Gear Head said:
Just one. Look up the satterlee method or a Chrono ladder.
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That is all well and good, but are you sure those velocities are true representations of what a load will do? I noticed a load or two that had reduced velocity with increased powder. My experience with chronographs and test loads has been that the same load may show differences in velocities that are greater than the differences among different loads. I am glad for Mr. Satterly that his method works for him, but I would feel more comfortable with at least 3 data points for each load.
 
Charlie said:
That is all well and good, but are you sure those velocities are true representations of what a load will do? I noticed a load or two that had reduced velocity with increased powder. My experience with chronographs and test loads has been that the same load may show differences in velocities that are greater than the differences among different loads. I am glad for Mr. Satterly that his method works for him, but I would feel more comfortable with at least 3 data points for each load.
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I figured I'd give it a try. The whole principle is to find a node where slight charge variances don't have much of an affect on velocity resulting in what should theoretically yield a group that gives low SD/ES. From here I'll load up about 10 at 41.0 and another 10 at 41.7 and check ES/SD and see how they group. If results are inconclusive then so be it but I thought it was a pretty cool concept that saves you from loading 5 at each charge and blowing through a bunch of supplies finding a load.
 
Gear Head said:
So I was using the sporter which comes with a big and small shim. I was using the small shim and was 3/8" off the centerline of the bore which was the max distance recommended on the paper in the box. I was unsure if I was supposed to run it with no shim at all but that would get me about 1/4" close which would leave me about 1/8" off the centerline. Next time I get it out I'll try to get it closer and see if that makes a difference.
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I'm unsure if the Sporter has this but the full Magnetospeed has a 1/4" square guide you lay on the MS bayonet to check distance from bore. You want about a 1/16" gap between the bore and the gauge.

Also, the MS has several different sensitivity settings and modes. Something could have been off perhaps.
 
Bunsen said:
I'm unsure if the Sporter has this but the full Magnetospeed has a 1/4" square guide you lay on the MS bayonet to check distance from bore. You want about a 1/16" gap between the bore and the gauge.

Also, the MS has several different sensitivity settings and modes. Something could have been off perhaps.
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Sporter does not. I was measuring to center of bore which was incorrect; supposed to measure to bottom of bullet path which is 1/4" for me and is well within the recommended distance. I had sensitivity on high 1 which I was told was correct for 6.5 creedmoor.
 
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