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Standard Deviation

konamtbikerkonamtbiker Member Posts: 284 ✭✭✭
edited March 2008 in Perry Shooter Competition Shooting and Reloading Forum
To my understanding the lower standard deviation you can get out of your ammo the better. Today at school an instructor said that standard deviation does not help out on accuracy? Want some other opinions on the matter. Thanks
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Comments

  • perry shooterperry shooter Member Posts: 17,105 ✭✭✭
    edited November -1
    Standard Deviation is ONE of many things that go into loading good ammo . but like a chain the weakest link is what is key. S.D of a very small amount is great but if it does not match the bullet to the sweet spot in velocity then group size will suffer . As an example a ten shot group with 230 grain ball in a standard twist 1911 45ACP barrel and a S.D. of say 10 FPS at 700 FPS most likely will not shoot as good of a group as the same bullet in the same barrel with a a S.D of 40 in the 800 FPS range.
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  • bpostbpost Member Posts: 32,669 ✭✭✭✭
    edited November -1
    Some of the best groups I ever shot with my Encore 6MMBR had a horrible SD. IIRC the SD was 38. The group was .238. From my experience, (somewhat limited) the SD is an indicator of potential accuracy rather than a target to acheive. If the SD is very low and the group looks like a shotgun pattern what use is the low SD?
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  • dtknowlesdtknowles Member Posts: 810 ✭✭✭✭
    edited November -1
    The correlation between SD of velocity and accuracy is low as practiced in shooting circles. First the SD is calculated from statistically small samples you would need to burn up a lot of ammo to get good SD data. Since we measure accuracy as group size with is the same as extreme spread you can't expect a strong correlation. I have not seen it studied but I would think that there would be better correlation between velocity extreme spread and group size.

    Tim
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  • sandwarriorsandwarrior Member Posts: 5,453 ✭✭✭
    edited November -1
    konamtbiker,

    I think this instructor needs to make a popping noise. SD = average constant velocity. The farther out you shoot the more it matters. It's a simple matter of physics, slow bullets drop faster than fast bullets. Like perry said though, the average still has to correlate to the sweet spot of the firearm. Having bad harmonics by running the bullet 100 fps too slow will send the extreme spread all over the place. They correlate to one another but each has to match it's intended target range.

    If this is regarding how much time you spend prepping brass and carefully working up loads to get everything to shoot just right, then you need to decide what your accuracy needs are. A lot of 100-200 yd. bench shooters don't weigh loads. They have a good measure and can throw loads that are within 1-2 tenths of a grain. That little difference isn't going to make a difference with accuracy at those short distances. Not nearly as much as having a load that doesn't harmonically match the barrel. Long range shooters can't afford to have bullets dropping out of the circle though. and loss of velocity by 50-100 fps will make a big difference.
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  • nononsensenononsense Member Posts: 10,928 ✭✭✭✭
    edited November -1
    konamtbiker,

    Standard Deviation is one small part of the whole picture. It helps tell you how consistent your ammunition is. But even this can be suspect depending on the conditions under which the data is collected. Inexpensive chronographs might not be ideal for collecting this piece of information. Sunlight or lack thereof, poor screen quality and dozens of other variable can have an impact on just this one number, so it can be suspect.

    The reason that this number is not a strong indicator is because there are other variables that can have a greater impact on accuracy. One of the biggest for long range shooting is the wind. When shooting at 600 through 1,000 yards, the wind can switch more than once at different distances. Put yourself at the bench and think about all of the things that can change your accuracy when shooting at targets. Standard Deviation is just a part of the picture. It deserves consideration but it isn't the end all.

    How's school going?

    Best.
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  • konamtbikerkonamtbiker Member Posts: 284 ✭✭✭
    edited November -1
    Thanks for all the input. I now have a great understanding of Standard Deviation. We covered it more in depth today at school. School is awesome. I am in my 4th month and am in Machine Shop. I have learned how to Taper barrels, Cut crowns, Install muzzle brakes, Learned how to Thread, I am very close to Chambering actions, and correcting headspace. Thanks for asking Nononsense.
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  • konamtbikerkonamtbiker Member Posts: 284 ✭✭✭
    edited November -1
    PS. I am still going to load for a good SD but if I am getting 1/4 or even 1/8 MOA at 100. With a SD of 17. Im not going to try to get it down to an 8 SD where my MOA could go south. Im sure theres guns out there that will shoot great with "bad" sd. Thanks again.
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  • SDefenderSDefender Member Posts: 4 ✭✭
    edited November -1
    Try this document for more info:

    http://www.shootingsoftware.com/ftp/Perverse Nature of SD.pdf
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