BS-GROUP SIZE AND ERRORS

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  • Last Post 24 August 2018
joeb33050 posted this 11 August 2018

 

GROUP SIZE AND ERRORS

 

Group size is the result of a set of what we will call “errors”. Errors might include shooter ability or primer choice.

 

As we eliminate errors, group size is reduced, as the square root of the sum of the squared errors.

 

If group size = 1”, and the perfect shooter reduces group size by .5 inch, and the perfect primer reduces group size by .5 inch, the combination reduces group size by (.5^2 + .5^2) ^.5 = (.25 + .25) ^.5 = .5 ^.5 = .71 inch, and the group size is 1 -.71 = .29 inch.

 

If group size = 1”; and we can eliminate 10 errors, each of which reduces group size .1 inch, the combination reduces group size by .1^2 = .01, X 10 = .1, .1^.5 = .32 inch, and the group size is 1 - .32 = .68 inch.

 

Reducing or eliminating small errors has a smaller effect on group size.

 

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frnkeore posted this 11 August 2018

Are you saying that shooter ability and primer choice are .1" errors?

Frank

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RicinYakima posted this 11 August 2018

No, it is a theoretical and hypothetical questions beginning with "If". "If" they were able to reduce group size by that amount, they give you less yield for the amount of effort required.

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Ross Smith posted this 12 August 2018

I like that also Joe, I really assumed that these things were additive, ie .5+.5=1   1-1=group size 0

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Ed Harris posted this 21 August 2018

I do all of my test shooting firing ten-shot groups, both rifle and pistol.  My feeling is that there is no such thing as a "lucky" ten-shot group, so while I might also shoot those rounds over the chronograph so that I know if the ballistic uniformity is good, more than anything else I want to see if the group is round, with a dense center and that the distribution of the shots is circular-normal.

I don't do ladder testing, but shoot ten-shot groups on paper over a range of charge weights, starting on the low end and in a case the size of the .3-'06 increase in full grain increments, one ten-shot group at a time. An iron-sighted '03 Springfield, Winchester 54 or Model 70 should shoot 2-inch or smaller 10-shot groups off sandbags at 100 yards if the load is working.  If the group is round, but loose, over 2-1/2" I look at bore condition for leading and check bullet fit.  Next step is to adjust bullet diameter and hardness, usually a bit fatter, softer and slower.  I then try again.  If a load shows promise under 2" I'll try again also bracketing the charge +/- 1/2 grain either way.  If the bullet holes are not round I will increase the charge a bit and try again.  #311299 is stable at subsonic velocity in a ten-inch twist.

I'm not a big fan out of frequent cleaning if a load is working well.  If trying a new load, lube, bullet, alloy or powder I'll run a dry patch through the bore after 20 shots, feel how it goes, examine the patch.

K.I.S.S. principle. 

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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joeb33050 posted this 21 August 2018

I do all of my test shooting firing ten-shot groups, both rifle and pistol.  My feeling is that there is no such thing as a "lucky" ten-shot group, so while I might also shoot those rounds over the chronograph so that I know if the ballistic uniformity is good, more than anything else I want to see if the group is round, with a dense center and that the distribution of the shots is circular-normal.

 

7.15.2 WHAT DOES "IN CONTROL" MEAN? 

 

In order for accuracy testing to yield true results, the load must be "in control". (For the purposes of this discussion, “load” includes the gun, sights, rest and all components such as powder, primer, bullet, etc.)

 

            The term "in control" means that the test results come from one distribution and not two or more distributions. If we test with two different bullets or lubes or powders or scopes at one time, then we are testing two loads, and will see what appear to be big groups. Maybe we're shooting small groups with each load, superimposed on each other making a big group.

 

For example, groups with three holes in one place and two holes in another place or groups that are taller than they are wide are symptoms of the load not being "in control".

 

Groups shot with loads that are "in control" are round, on average, in the long run.

 

It is difficult to impossible to determine if one or two groups are round by just looking at them. Size doesn't come into it; big groups may be "in control" and small groups may not.

 

One way to find out if your loads are "in control" and if your test results are true is to shoot a lot of shots into one target The group of a lot of shots will be pretty round if the load is "in control".

 

Another way is to shoot a number of groups using a backer. Put a piece of paper or a target on the target frame, put a target on top, and shoot. Then remove the record target, align another record target on the backer and shoot that record target. Keep this up, noting pulled shots on the record targets. When done you'll have the set of record targets and the backer with all shots fired at the record targets. Note the pulled shots on the backer. The holes in the backer, less the pulled shots, should form a roughly round group.

 

 Groups that appear to be not in control are a sign that something is amiss with the load. Let the search for the culprit begin.  

 

 

 

I don't do ladder testing, but shoot ten-shot groups on paper over a range of charge weights, starting on the low end and in a case the size of the .3-'06 increase in full grain increments, one ten-shot group at a time. An iron-sighted '03 Springfield, Winchester 54 or Model 70 should shoot 2-inch or smaller 10-shot groups off sandbags at 100 yards if the load is working.  If the group is round, but loose, over 2-1/2" I look at bore condition for leading and check bullet fit.  Next step is to adjust bullet diameter and hardness, usually a bit fatter, softer and slower.  I then try again.  If a load shows promise under 2" I'll try again also bracketing the charge +/- 1/2 grain either way.  If the bullet holes are not round I will increase the charge a bit and try again.  #311299 is stable at subsonic velocity in a ten-inch twist.

I'm not a big fan out of frequent cleaning if a load is working well.  If trying a new load, lube, bullet, alloy or powder I'll run a dry patch through the bore after 20 shots, feel how it goes, examine the patch.

K.I.S.S. principle. 

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GWarden posted this 22 August 2018

I have ended up doing the same as Ed, 10 shot groups. Learn more from those groups and about my being consistent in proper holding.

Bob

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Glenn R. Latham posted this 22 August 2018

GWarden wrote: "Learn more from those groups and about my being consistent in proper holding."

Amen to that!!

Glenn R. Latham

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Ed Harris posted this 22 August 2018

Joe, I read your post three times, and this hillbilly can't figure out what you are asking.

My method was taught to me at Frank Marshall's knee as a kid.  It works.  I never saw any reason to change.

I don't care about scientific explanations about why or how it works, I just know that it does.

Period.

 

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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John Alexander posted this 22 August 2018

Joe,

Thanks for posting your initial post. If more shooters understood how very insignificant small errors are when there are bigger errors involved, maybe we would feel more comfortable easing up on the obsessive little details that we hope will improve accuracy but have no proof.

It applies directly to the argument I am making in the neck tension thread with limited success.

John

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dbarron posted this 23 August 2018

Quite successfully, I'd say. Although you do appear to be trying to confuse some people with facts.

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Ken Campbell Iowa posted this 23 August 2018

dbarron ...

heh, even real serious experimenters such as joeb .... get to have a little fun once in a while...

we just have to stay alert !!

ken

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dbarron posted this 24 August 2018

Ken: Roger that.

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