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Chronograph variance, between make/model


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Do you notice variance in readings from one chrono to another? I am curious if there is much difference, and the reason I ask is because of barely making major power factor at Area 4 last year, like by the skin of my teeth. I have friends who stay 6-8 PF above floor for either major or minor, and it is like insurance for major matches.

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Yes, there are variances all over the place.

I know the Desert Classic uses two chrono and their velocities have never match as far as I have seen over the last 8 years.

I believe they use two at all the Nationals also.

Unless you are using bench rest reloading equipment you are still going to get variable velocities just not as much.

I can cut my velocities pretty close for the Desert Classic and Nationals because I load with VV320 and have my chrono records and match chrono slips as well as the match ammo so I can always compare the results when changing powder lots. I take three match rounds to establish a base line then confirm my latest load chrono to that ammo.

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As a chronographer at major matches in the east, I recommend for a 125PF go 133, 150PF go 160, 165PF go 172.

You can go better, but shouldn't go less.

FWIW I run 137 for minor and 177 for major. Never an issue.

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  • 2 weeks later...

I chronoed consistently 131-132pf on my chrono last year. Using same 8lb jug of powder on every load. Shot matches from Charleston,SC to Barry, IL and ranged from 126.4-137pf. So many variables play part in the chrono of ammo. I stay atleast 6pf over forbthe just incase. I would hate to drive 4hrs or more to a Level 2 and not make pf. Seen a few friends make that trip and it sucks!

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Folks describe the mysteries of the numbers at Chrono as Voodoo for good reason.

It does not matter if your load is ALWAYS 127 PF or 167 PF with extreme spreads and standard deviations in the single digits at your home range with your very own Oehler. Elsewhere, changes in temperature and elevation may make a difference, and differing or grumpy electronics even more of a difference.

I go 8 to 10 PF points over the Production PF floor. Initially that was because my Glocks seemed to prefer it, but as a result I have not had to sweat Chrono in years (I take that back - I had a gun problem one year but I managed to fix it legally).

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The electronics in most chronos are pretty accurate, however, they work by timing the appearance of the bullet over the start and stop sensors. The critical dimension for accurately reading velocity correctly is therefore the distance between the sensors. Most of us just assemble the chrono and use it without ever checking this distance, and variances in the manufacturing of the bar that the sensors mount to can account for some of the differences we see between different makes, and even between two chronos that are the same make and model. If you want the most accurate reading possible with your equipment, always bring a tape measure to the range with your chrono and check the distance between the sensor windows. I use a CED M2, and after getting some funky readings one outing at the range, I realized I tightened down the stop sensor 1/8" before it was fully seated. I now always bring a tape measure to ensure I always assemble the thing with 24" between the sensors.

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  • 3 weeks later...

For an explanation of why you shouldn't reload based on adding a few PF points, see page 70 of Jan/Feb 2015 edition of Frontsight magazine. As I explain, you should use the standard deviation (STD) instead. Add at least 2 times your STD to the minimum velocity to make PF. Use 2.5 or even 3 times depending on the unknowns. I think you will fing the example I give helpful.

Shoot with any questions.

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As Kevin c pointed out, there are a lot of factors that affect velocity and they do so far more than chrono accuracy. And ALL of them are random in nature. Thus, the best way to manage them is to know and use your STD to ensure you meet PF.

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  • 2 months later...

For an explanation of why you shouldn't reload based on adding a few PF points, see page 70 of Jan/Feb 2015 edition of Frontsight magazine. As I explain, you should use the standard deviation (STD) instead. Add at least 2 times your STD to the minimum velocity to make PF. Use 2.5 or even 3 times depending on the unknowns. I think you will fing the example I give helpful.

Shoot with any questions.

Thanks! I was just planning on hunting down that article again...

it was very useful.

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As Kevin c pointed out, there are a lot of factors that affect velocity and they do so far more than chrono accuracy. And ALL of them are random in nature. Thus, the best way to manage them is to know and use your STD to ensure you meet PF.

And include the bullet weight SD!

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Do you notice variance in readings from one chrono to another? I am curious if there is much difference, and the reason I ask is because of barely making major power factor at Area 4 last year, like by the skin of my teeth. I have friends who stay 6-8 PF above floor for either major or minor, and it is like insurance for major matches.

Forget about adding PF points, focus on the minimum average velocity needed to make PF and your STD. Its important to understand that there is a huge difference in measuring your velocity using 8 or 10 rounds at the range versus only 3 rounds used in the official PF measurement. You cannot directly compare the two as they are apples and oranges. That's the whole point of my article. Using the table of z values, you can now relate your measured velocity at the range to the official PF test but only via a confidence level.

For more of an explanation see this post and the example on 25Feb at about 9AM.

http://www.brianenos...opic=229005&hl=

http://www.brianenos...opic=229005&hl=

Edited by jwhittin
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  • 2 weeks later...

So I got to test my Caldwell chrono the other night - using the IR lights - was indoors.

Everything worked fine - almost too easy - using my iphone to record data.

I ran some WWB first to get a baseline - this usually produces about 134-135 PF out of my g34

I also ran some of the last batch I did on my 550, and a couple of batches from new 650,

the 2nd load having very slightly higher charge (because I was too lazy to fiddle with powder measure ;-)

All 8 rounds though g34

WWB 115gr ?? ?? avg 1233.4 SD: 24.5

550 124gr 4.8gr WSF avg 1088.4 SD: 24.7

650 124gr 4.8gr WSF avg 1102.2 SD: 17.9

650 124gr 4.85gr WSF avg 1105.4 SD: 15.6

all wonderful, except that avg speed for WWB is high by probably 50fps compared to what

I've seen at matches, but there the temp is probaly 70-80 degrees compared to 60 the other night.

Interestingly the only time the above load has been chrono'd at a macth ISTR the avg speed being as above about 1088 and made PF 134 or 135.

Question is, can I usefully relate my results above to what I can expect at a match (same area).

Edited by randomBytes
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  • 3 months later...

RandomB

The connection between the two is quantified by the confidence level of passing PF which corresponds to the value of Z you use (see article above).  So in your case, if the conditions when you checked you ammo and when it was tested for PF are the same, then its reasonable to use Z=2 and add 2*STD to the velocity required to meet PF for you bullet weight. Selecting Z=2 gives you about a 90% confidence that you will pass PF using the first 3 rounds of the PF test.  If there were unknowns and you wanted more margin, then increase Z.  Z=2.5 bumps up the confidence level to about 95%, Z=3 is about 97%. 

Your 650 load data looks good. Maybe a tad hot unless you want the extra margin.  If I use z=2 I get 1044 or higher as a load velocity for a min 90% confidence.  Again, that's for all things equal.  Adjust from there.  

 

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  • 3 weeks later...

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