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Rifles, Reloading, Optics, Equipment
Reloading
SD how many rounds.
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<blockquote data-quote="Buffalobob" data-source="post: 108705" data-attributes="member: 8"><p>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.</p><p></p><p>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.</p><p></p><p>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.</p><p></p><p>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.</p><p></p><p>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.</p></blockquote><p></p>
[QUOTE="Buffalobob, post: 108705, member: 8"] 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. [/QUOTE]
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SD how many rounds.
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