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Interpreting Chrono Results.


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Ok, I've got the chrono and I've checked my loads. What am I supposed to be looking for?

What is an acceptable "extreme spread" and "standard deviation"?

How can I influence either of those measurements?

As best I can tell they are a function of my Dillon powder measure.

I'm loading 40 caliber, 180 gr JHP (Zero's at the moment), and am using mixed brass, fired once or twice. I know I can clean things up by using only one brand of brass, but that's just not an option right now.

Also, is it just the average velocity that needs to make Major? Or should all shots in a string be above Major?

I just need to know what I should be striving for now that I have a chrono.

Any help at all is greatly appreciated.

Also, is there a picture of what primer flowback looks like? I can't find it.

-Cuz

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Also, is it just the average velocity that needs to make Major? Or should all shots in a string be above Major?

In USPSA, you will be asked to provide 8 bullets. One will be pulled and weighed. That leaves seven. When you get to chrono, three shots will be fired and averaged. If these make the desired power factor (you imply major), you're golden. If they do not, three more will be fired. The power factor will be calculated using the 3 highest of the 6 rounds. If this average makes the desired power factor, golden. If not, you can either have the 7th round weighed (hoping it weighs enough more than the first one pulled - usually a bad choice) or fired (hoping it has a higher velocity than your third highest of the first six and that this is sufficient to make major).

When testing my loads, which I do very regularly, I fire six shots. As long as all six exceed the needed FPS, I'm comfortable with the loads. I target between 167 and 168 PF. I have never failed to make major power factor using this system, although I came very close once. I KNOW it was the chronograph. :)

Edited by davidball
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My "normal" standard deviation is 4-10 for 40 cal major Limited loads with mixed brass and 6-12 in 9mm major Open loads with mixed brass. My minor 9mm loads run 4-6.

You can drop them down using one brand of brass, good metering powders, smooth press operation, and an EGW U die to get consistant case tension.

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Making Major, I want an avg. of 170. No round should be under 165pf.

Been running the same load for 5 years. I go to chrono with confidance. Heard lots of whining about "slow chrono's" at some matches. No problem for me.

You won't be able to tell the difference between 167, and 170pf, until you get minored. <_<

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My "normal" standard deviation is 4-10 for 40 cal major Limited loads with mixed brass and 6-12 in 9mm major Open loads with mixed brass. My minor 9mm loads run 4-6.

You can drop them down using one brand of brass, good metering powders, smooth press operation, and an EGW U die to get consistant case tension.

How many strings are you measuring this across? How many rounds per string?

I ran 5 strings of 10 rounds each. My Standard Dev. runs between 10 and 22. My Extreme Spread runs between 37 and 84. Is that "normal" for a load using mixed brass?

I don't want to have to measure every charge. With Titegroup powder my 550B powder measure should be pretty accurate and relatively consistent. Although I do notice quite a drop off depending on how much powder is in the hopper. I try to keep it around 3/4 full all the time.

-Cuz.

Making Major, I want an avg. of 170. No round should be under 165pf.

Been running the same load for 5 years. I go to chrono with confidance. Heard lots of whining about "slow chrono's" at some matches. No problem for me.

You won't be able to tell the difference between 167, and 170pf, until you get minored. <_<

Are you shooting strings of 10 rounds? Are you shooting a 40 or 45? With my limited gun in 40 cal and a 180gr bullet it's tough to average 944fps (170pf) and have no rounds dip below 917fps (165pf). Plus, then I get some rounds up near 1000fps which seems dangerous. I've checked some of my fired casings and some of the primers have a small ring around the indent from the firing pin. Is this a sign of high pressure?

-Cuz.

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Making Major, I want an avg. of 170. No round should be under 165pf.

Been running the same load for 5 years. I go to chrono with confidance. Heard lots of whining about "slow chrono's" at some matches. No problem for me.

You won't be able to tell the difference between 167, and 170pf, until you get minored. <_<

Rich - Ditto. I'm working up a new minor revolver load this summer, and it's not only going to make a 130 PF, it's going to make it from my slowest revolver. It'll make about 140 from my faster ones - I won't be able to tell the difference, with the possible exception of poppers falling a hair more quickly. ;)

Edited by revchuck
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...I get some rounds up near 1000fps which seems dangerous....

-Cuz.

IIRC, nominal specs on the 40 S&W round, loaded to SAAMI standards, are for around 950 fps for a cartridge holding a 180 grain bullet with a COL of 1.135".

The old PF requirements for 175 PF had folks loading 180 grainers to 1000 fps, loaded long in most cases.

So you're not necesarily pushing the envelope much in terms of pressure, assuming you load long and use a slow powder. There can be an issue for shorter OAL combined with heavy slugs and very fast powders (the Clays load over a 200 grainer at 900 fps was the hot load in the 175 PF days, literally... :wacko: ).

Still, a smaller ES would be nice confidence wise when going to the chrono stage. Note that I did not say anything about accuracy - while consistent ammo (small ES) has greater predictability on chrono readings, it may or may not be accurate, and ammo with fairly wide swings in velocity can still be tack driving fodder at pistol ranges.

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  • 10 years later...

Cuz,

To answer your original questions, the extreme spread is not really useful for reloading (you can't use it to make any useful conclusions about your reloads). For meeting PF, you should ALWAYS use the Standard Deviation NOT the average velocity converted to PF.   You can read the article to explain how to use your SD.  Its very simple.  

You simply add at least 2 twice your measured SD to the minimum velocity required to meet PF for your bullet.  This is now your desired average velocity for your loads.  So a 180 gn bullet requires 917 FPS to make major.  If your SD = 20 then 917 + 2*20 = 957 fps.  That's the average velocity you should load for. You can use 2.2, 2.5 or even 3 time your SD as needed for unknown environmental differences or to adjust for reliability and accuracy.  As people have pointed out, you won't be able to tell the diff between 168 and 172 PF. 

This method gives you a high confidence of passing an official PF check using only your first 3 rounds so you don't have to sweat it!

From my experience, most people end up with SD measurements in the 10 to 20 range. That's very reasonable. I shoot mixed brass 45 ACP and my current reloading process gives me a consistent 15 fps. Some people are able to consistently get below 10 fps.  Using the method above, it doesn't matter as long as you check your SD from time to time.  

Don't listen to the people saying you should use the ES or measure 20 rounds, or no velocity should be below X, that's all a waste of time and money.  You only need to use 8 to 10 rounds to check your SD and velocity.  

Hope this helps.  Shoot with any questions.

 

 

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  • 5 months later...

Jwhittin - Thanks for this info and the link to the other post.  I'm sure I would have gotten to that post eventually, but it is nice to have it right here. :) 

 

I'm about to buy a chrono and I'm planning on loading .40 for Major with 180gr bullets as well (similar to the OP), so it is good to get this info now before I'm loading up my rounds and I start to test them on the chrono when I get it.  This way I will know what to look for regarding the SD and the avg SD I should hope to achieve with my loading process.

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On ‎5‎/‎28‎/‎2006 at 0:22 PM, wide45 said:

Making Major, I want an avg. of 170. No round should be under 165pf.

 

^^^^    I agree     ^^^^^^    

 

I test 20 rounds over the chrono, and I want them ALL to hit 165    :) 

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