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SD how many rounds.

 
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  #1  
Old 05-06-2006, 02:18 PM
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SD how many rounds.

hi all
how many rounds do you have to put through the chrono before you can get a reliable SD.

i did 3 bullets today,then it through it down with rain.
so i packed up.and i am out tomorrow if there is no rain.

on the 3 i put through my 243 AI shooting 95g VLD with 49.6g of n165.i got this reading.

pact chrono xp

high 3273.2 fps
low 3270.6 fps

SD 1.3 fps
CV 0.0%

mad 0.9 fps
CV 0.0%

av 3271.9 fps
es 2.6 fps

shot 1 3273.2 fps
shot 2 3270.6 fps
shot 3 3271.9 fps

and then like i say i had to run for cover.
so i am wondering how many should i load to get a good useful read out
thanks for all your continued help
Colin
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  #2  
Old 05-06-2006, 09:59 PM
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Re: SD how many rounds.

I would say that 3 would be good if your cases are quality , if you run say 10 through and you get one or two that blow the curve then chances are that their was a foul up in the loading or the cases are drasticaly differant than the rest.

have you had any better luck getting those Bergers to group?
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  #3  
Old 05-07-2006, 02:19 AM
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Re: SD how many rounds.

[ QUOTE ]
I would say that 3 would be good if your cases are quality , if you run say 10 through and you get one or two that blow the curve then chances are that their was a foul up in the loading or the cases are drasticaly differant than the rest.

have you had any better luck getting those Bergers to group?

[/ QUOTE ]

i have gone up to 10 thou of jam 2 4 6 8 and 10 thou and on the lands,
now 2 thou shot a .5
4 thou shot a .6
6 thou shot a .280
8 thou shot a .8
and 10 thou shot a.180

so i am off out today to shoot 10 thou and 6 thou again
going to make up 10 off each and if i don't get any consistency i am going to bin the 95g VLDs.and go and get some 90g Berger's,i don't like giving up but some times you have to stop banging your head against a wall. [img]/ubbthreads/images/graemlins/confused.gif[/img]

if the weather out today is better,i will run them through the chrono just to torture myself some more [img]/ubbthreads/images/graemlins/shocked.gif[/img] [img]/ubbthreads/images/graemlins/confused.gif[/img] [img]/ubbthreads/images/graemlins/grin.gif[/img]
thanks
colin
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  #4  
Old 05-07-2006, 05:53 AM
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Re: SD how many rounds.

Statistically speaking, it depends on what confidence level (chances of equalling or bettering it the next time you test) you want. If you want to be sure that the SD you come up with is correct 90% of the time, you'll need to shoot at least 15 to 20 shots per test. Fewer shots fired lowers the confidence level. With only 3 shots per test, there's not much confidence in it and it's easy to get a larger number.
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Old 05-07-2006, 10:52 AM
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Re: SD how many rounds.

Three shots will work to get an idea but then test the load again and again (on seperate days in differing weather conditions) in three shot groups to verify that it still works. If the sd goes up or down 1 or two feet per second over the course of the re-test, then it will work well for you.
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  #6  
Old 05-07-2006, 11:28 AM
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Re: SD how many rounds.

As Bart B says the standard deviation of three rounds has no mathmatical meaning. You can look at the extreme spread and if it is not satisfactory then you will know that there is a low "probablity" that the SD will be satisfactory to you. If the ES is satisfactory to you that is not - I repeat not- mathematical proof that the SD will be satisfactory.

How many rounds you have to fire to get an ES depends upon the end application and what level of "confidence" you want. If you are just shooting pdogs at 300 yds and don't mind a miss every now and then maybe 3 rounds is enough. If you are shooting 1000 yds benchrest with 10 rounds of record and you desparetely want to win then you will need to shoot about 30-40 rounds to guarantee that the velocity is consistent. Not all of these round need to be fired at a sitting. You can collect the velocities over the course of several shootings and enter them into a statistics program. Secondly, you do not want the SD, what you want is 2 or 3 SDs. One SD tells you that two thirds of your bullets will be traveling within this velocity range and that would mean that four bullets out of the 10 you fire for record would be outside that velocity range. Two SDs tells you that 95.46% of 10 bullets will be with the velocity range. If you win your relay and have to fire a second ten bullets then one bullet will be outside of the velocity range. Three SDs tells you that 99.7% of your bullets will have a velocity within that spread. In other words, you have confidence that your 10 rounds of record will all be with +/- that velocity spread. If three SDs cause your groups to be too large then you need to go back to load developement. If your ego will allow you to lose then you can go with less.

Put simply you must have more "data points" than you will shoot in order to have very very good faith in the "statistical probability". Just shooting milk jugs you don't really need much data.

For those of you that have never had to sit in a boring statistics class on a nice spring day, the way you get 2 SDs is to simply multiply the SD that comes off your chronograph by two (or three if you want 3 SDs) This will tell you that all of your velocities will be within plus or minus this amount.

For all of the people who get really wigged out - I would suggest you check the error induced by a wind measurement versus the error induced by a bad bullet velocity. At long range I would bet that your ability to measure the wind will induce more error that some sloppiness in measureing SD.
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  #7  
Old 05-07-2006, 12:32 PM
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Re: SD how many rounds.

thanks for all your help everyone.
and 3 x 15 shot groups it is then,i don't like to miss, [img]/ubbthreads/images/graemlins/smirk.gif[/img] but i do often [img]/ubbthreads/images/graemlins/blush.gif[/img]
and i don't shoot pd i will be shooting crows and fox.
thanks
Colin
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