Acceptable standard deviation
- Thread starter Jrji
- Start date
Unless you are shooting long range, the SD does not matter much.*
For long range shooting (800 yards and beyond) , I like single digit SDs.
Use brass from the same lot# that was been weighed, shot the same number of times, with uniform neck tension. Use a quality powder. For 223 N135 or N140 is excellent. For 80 gr bullets, N150 works.
* For close range shooting, just develop a load that groups well in your rifle at the distance you want to shoot.
For long range shooting (800 yards and beyond) , I like single digit SDs.
Use brass from the same lot# that was been weighed, shot the same number of times, with uniform neck tension. Use a quality powder. For 223 N135 or N140 is excellent. For 80 gr bullets, N150 works.
* For close range shooting, just develop a load that groups well in your rifle at the distance you want to shoot.
What is more important, accuracy or some random velocity value?
People look at SD because they believe it will indicate long range accuracy and there may be some truth to that. However the only thing that actually matters is how the load groups on the target. Everything else is secondary.
Years ago I spent hours and hours sorting ammo by bullet runout. In the end I had a pile of the unacceptable ammo. When I shot that pile, it grouped better than all the stuff I had sorted into perfect groups. Since then I have never bothered to sort ammo or measure runout.
People look at SD because they believe it will indicate long range accuracy and there may be some truth to that. However the only thing that actually matters is how the load groups on the target. Everything else is secondary.
Years ago I spent hours and hours sorting ammo by bullet runout. In the end I had a pile of the unacceptable ammo. When I shot that pile, it grouped better than all the stuff I had sorted into perfect groups. Since then I have never bothered to sort ammo or measure runout.
I presume you are talking about velocity? To answer that question correctly you really need to know the width of your velocity sweet spot as determined by a ladder test. Once you know the velocity that puts you in the center of that band of acceptable velocities, then you can calculate your probability of being within that band by adding and subtracting 1, 2 and 3 standard deviations to the center value. See the graph below. If your band for example is from -1 standard deviation to +1 standard deviation, then your chances of being within that range for future shots is 68%. If your band is from -2 to +2 standard deviations, then your chances of staying within it is 95.4%. The standard deviation of 5 shots is rather meaningless. You need the standard deviation of at least 20-25 shots to get statistically valid data.
