Besides powder charge, what are the two most important reloading steps that will decrease SD?
Ben
Ben
Standard Deviation
- Thread starter BenPerfected
- Start date
shoot more bullets, the population will increase thus decreasing the standard deviation - dump 200 or so rounds in a big box then select say 20, subtract 1 (Bessel's correction) and use the value 19 as the population
reduce individual variations will also help to reduce standard deviation as the deviations are subtracted by average value then squared
select specific powders most suitable and of common use for the caliber -- don't try to re-invent the wheel
maintain uniformity in mechanical operations, brass dimensions, cartridge dimensions
reduce individual variations will also help to reduce standard deviation as the deviations are subtracted by average value then squared
select specific powders most suitable and of common use for the caliber -- don't try to re-invent the wheel
maintain uniformity in mechanical operations, brass dimensions, cartridge dimensions
For me it's charge weight accuracy and being in middle of node, #2 Consistent neck tension, #3 Primer selection.
Shoot two-round groups. It does wonders for SD and really improves MOA too.
SD is only a number the larger amount of numbers the less margin of error. Larry
Larger sample size will not give you a lower SD, it will give you a more accurate SD. Once you have a sample size of 10, you're pretty close to as accurate a number as you're going to get.
Agree with all the above comments about neck tension, brass prep, finding the right node, seating depth, etc. Other thing I'll add is trying out different primers. I tried CCI 200's, Fed 210M's, and BR-2's recently. 10 shot sample size with each, all else held equal. CCI200 gave me an SD of 13, 210M was 11, and the BR2's gave an SD of 5.
I don't think this is because the CCI200's or 210M are bad, it was just the interaction with this particular load and gun.
Lapua40X
California Hunter Education Instructor
Ben,
I don't know why you're heavily focused on SD but I would add this to what's been offered previously.
A low SD does not necessarily represent good accuracy. SD is simply a product of the comparison of muzzle velocity samples (ES) and, while shots put on target with a consistently low ES do tend to be more accurate, that accuracy may not survive over longer ranges.
I typically find a load that prints well on target at 200 yards with a seating of about .010 - .020 off the lands. Some shooters start on the lands because they feel it's easier to move the seating depth in one direction than to take the chance that the node might be closer to the lands than .010. But that's a matter of style based on their experience. With my load pretty well established, I then work with seating depths to find a node (or two) and fine tune the one that offers the tightest MV range and work toward an SD below 10.
I don't know why you're heavily focused on SD but I would add this to what's been offered previously.
A low SD does not necessarily represent good accuracy. SD is simply a product of the comparison of muzzle velocity samples (ES) and, while shots put on target with a consistently low ES do tend to be more accurate, that accuracy may not survive over longer ranges.
I typically find a load that prints well on target at 200 yards with a seating of about .010 - .020 off the lands. Some shooters start on the lands because they feel it's easier to move the seating depth in one direction than to take the chance that the node might be closer to the lands than .010. But that's a matter of style based on their experience. With my load pretty well established, I then work with seating depths to find a node (or two) and fine tune the one that offers the tightest MV range and work toward an SD below 10.
I am not focused on std div and seldom use a chronograph. Based on the responses, it seems like the Std Div is going to be mostly about the brass prep assuming the powder charge & seating depth are consistent to perfect...correct? Day in and day out, what Std Div would you expect from 50 shots of your normally loaded ammo? Besides neck thickness and maybe seating force in lbs, how important is the shoulder bump to consistency in accuracy?....if the shoulder has some spring back so you can feel some resistance when closing the bolt, does this effect Std Div or accuracy?
Ben
billlarson
"Hold Into The Wind"
+2......... aman
My thoughts are:
A smaller SD, less velocity variations thus less drop variations at long ranges.
Observations of good long range performance using ammo having larger SD's than having lower SD's are probably anecdotal in nature and require more documentation.
Looking at any small data set having an elevated SD would be influenced or skewed as only one relatively gross anomaly would affect the SD calculation and if more samples were included the SD might be lower - more samples more validity.
The extensive variation of rifle chamber dimensions and bullet ogives is extensive and seating depth assumptions are speculative.
When playing around with this stuff "Student's t-Test methods might be considered. Bullets, powder, rifle barrels, other components and time are expensive. The idea is to make an inference based on a small sample. The determined average and standard deviation might be different from what would be observed from a much greater sample. To get into this go on-line and find a "t-Table" showing rows and columns. The rows are id' as degrees of freedom (df) or number of cases - 1; the columns are id' as confidence level percent, individual values are "t values" for selected confidence levels. A 95 percent confidence level would indicate that 95% of the time the actual mean or average of the extended sample would be in a specified range using the following calculation.
The math is: actual mean = average (from small sample observation) + or - t value (from table for a selected confidence level) * SD (from small sample observation)/ square root of number of projected samples.
The results produced are low to high means for an extended or much greater populations. Excel does a great job with this and possibly a small portable tablet or number generator thingy could be used at the range. Possibly chronographs might have stuff like the "student's t" built into them.
"What SD's would you expect from 50 shots of normally loaded ammo/"
My parting thoughts are that neck wall/ chamber/bullet diameters, degree of jamb or jump, and powder selection for any given cartridge are most important (customary use of Varget and H4350).
It is true that primers have very small variations in explosive content and that "match grade" primers might exhibit less variation - their use is probably valid for minimum pressure variations.
A smaller SD, less velocity variations thus less drop variations at long ranges.
Observations of good long range performance using ammo having larger SD's than having lower SD's are probably anecdotal in nature and require more documentation.
Looking at any small data set having an elevated SD would be influenced or skewed as only one relatively gross anomaly would affect the SD calculation and if more samples were included the SD might be lower - more samples more validity.
The extensive variation of rifle chamber dimensions and bullet ogives is extensive and seating depth assumptions are speculative.
When playing around with this stuff "Student's t-Test methods might be considered. Bullets, powder, rifle barrels, other components and time are expensive. The idea is to make an inference based on a small sample. The determined average and standard deviation might be different from what would be observed from a much greater sample. To get into this go on-line and find a "t-Table" showing rows and columns. The rows are id' as degrees of freedom (df) or number of cases - 1; the columns are id' as confidence level percent, individual values are "t values" for selected confidence levels. A 95 percent confidence level would indicate that 95% of the time the actual mean or average of the extended sample would be in a specified range using the following calculation.
The math is: actual mean = average (from small sample observation) + or - t value (from table for a selected confidence level) * SD (from small sample observation)/ square root of number of projected samples.
The results produced are low to high means for an extended or much greater populations. Excel does a great job with this and possibly a small portable tablet or number generator thingy could be used at the range. Possibly chronographs might have stuff like the "student's t" built into them.
"What SD's would you expect from 50 shots of normally loaded ammo/"
My parting thoughts are that neck wall/ chamber/bullet diameters, degree of jamb or jump, and powder selection for any given cartridge are most important (customary use of Varget and H4350).
It is true that primers have very small variations in explosive content and that "match grade" primers might exhibit less variation - their use is probably valid for minimum pressure variations.
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Shynloco
You can lead a horse to water, but ........
AMEN +10! All the other opinionated factors mean zip if the paper doesn't show the results you seek.
Addition stuff on this:
The velocity data can be considered as a normal distribution or like that bell shaped curve having 2 tails and an upward curve. The extreme values, both low and high are limited in number and located on either tail of the curve. The bulk of the data will be on the top middle. This will result in less variance or lower standard deviations as more data is analyzed or increase of number of values because more of the data is in the middle range and not at either extreme end.
All this assumes reasonable consistency of the loading operation. Also, this assumes the short sample velocities had some big standard deviations that would indicated by very small or large variations found on the "tails" of the normal distribution.
Velocity variations are of a major consideration as shown by 1000 yard bench rest matches (great paper work like closely spaced bullet holes!). How many inches of drop will happen with velocity differences of 50 or more fps at 1000?
The velocity data can be considered as a normal distribution or like that bell shaped curve having 2 tails and an upward curve. The extreme values, both low and high are limited in number and located on either tail of the curve. The bulk of the data will be on the top middle. This will result in less variance or lower standard deviations as more data is analyzed or increase of number of values because more of the data is in the middle range and not at either extreme end.
All this assumes reasonable consistency of the loading operation. Also, this assumes the short sample velocities had some big standard deviations that would indicated by very small or large variations found on the "tails" of the normal distribution.
Velocity variations are of a major consideration as shown by 1000 yard bench rest matches (great paper work like closely spaced bullet holes!). How many inches of drop will happen with velocity differences of 50 or more fps at 1000?
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Lapua40X
California Hunter Education Instructor
Ben, it ALL inflouences SD and accuracy. It affects SD because SD is a factor derived from ES which will be determined by the balance of every other aspect of your reloading process. Your accuracy potential cannot be assigned to one specific aspect of that process.
My feeling that applying the "Student t" stuff might have some applicability in getting some idea what might happen when small sample stats were applied to bigger samples. Like avoiding chronograph measurements of 50 or more rounds.
For example:
velocities, fps
2950
3078
3024
3037
3023
3033
3021
3058
3067
3100
10 cases
expand to 1000 cases
3022.4 average
41.35 standard deviation
1.962 approximate t value 1000-1 or 999 degrees of freedom, 95% confidence -- from table
31.62 square root of 1000, no t value for 1000-1
3024.97 upper mean range - math shown on previous post
3019.83 lower mean range
running the same for 60 cases
3022.4 average
41.35 standard deviation
2.00 approximate t value 60-1 or 59 degrees of freedom. 95 % confidence -- from table
7.75 square root of 60, no t value for 60-1
3033.08 upper mean range - math shown on previous post
3011.72 lower mean range
This type of thing is commonly used for all types of stuff in business and science
I have not used this stuff yet for ammo making, BUT, and I load lots and lots of ammo used for long range varmint hunting - chucks, prairie dogs, gophers, coyotes - for my self and relatives, like 4000 (40, 100 bullet boxes) rounds per year. Hopefully, this stuff might provide an indication of what I might expect. Naturally, other factors enter into these events one of which is how good can you shoot. Avoiding big velocity anomalies goes some way into making lots of hits. IMR 8208 - .204R, .20 Practical and H4350 .22-250 (70-75 grain) and 6.5-06 100 grain are the powders used and these have provided uniformity in all temperature ranges. In a former life I used stuff like this to indicate where to search for things. At one time I shot matches up to 1000 yards and liked to load a seasons ammo at one time and this type of analysis would have helped to develop loads.
For example:
velocities, fps
2950
3078
3024
3037
3023
3033
3021
3058
3067
3100
10 cases
expand to 1000 cases
3022.4 average
41.35 standard deviation
1.962 approximate t value 1000-1 or 999 degrees of freedom, 95% confidence -- from table
31.62 square root of 1000, no t value for 1000-1
3024.97 upper mean range - math shown on previous post
3019.83 lower mean range
running the same for 60 cases
3022.4 average
41.35 standard deviation
2.00 approximate t value 60-1 or 59 degrees of freedom. 95 % confidence -- from table
7.75 square root of 60, no t value for 60-1
3033.08 upper mean range - math shown on previous post
3011.72 lower mean range
This type of thing is commonly used for all types of stuff in business and science
I have not used this stuff yet for ammo making, BUT, and I load lots and lots of ammo used for long range varmint hunting - chucks, prairie dogs, gophers, coyotes - for my self and relatives, like 4000 (40, 100 bullet boxes) rounds per year. Hopefully, this stuff might provide an indication of what I might expect. Naturally, other factors enter into these events one of which is how good can you shoot. Avoiding big velocity anomalies goes some way into making lots of hits. IMR 8208 - .204R, .20 Practical and H4350 .22-250 (70-75 grain) and 6.5-06 100 grain are the powders used and these have provided uniformity in all temperature ranges. In a former life I used stuff like this to indicate where to search for things. At one time I shot matches up to 1000 yards and liked to load a seasons ammo at one time and this type of analysis would have helped to develop loads.
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