Defining accuracy

A while back, I was pounced on because I stated that measuring bullet velocity and it's Standard Deviation, was not meaningful with 3 and 5 shot groups.
I recommended 32 shot groups. (been a long time since Stat Class in College)....
So, I did some research with Dr. Ken Oehler's topics concerning SD.... I have a Model 35P.

So, the good Dr. said trying to determine SD from 5 shot groups is "in the same league as trying to determine the accuracy with a 5 shot group". "Likewise, a SD number should be considered only as an INDICATION of uniformity".
He and other statisticians consider the acceptable sample size to be "20 rounds".

Now, I am the first to admit that every time I put a rifle/scope together, if it shoots a 5 round ragged hole at 50 or 100 yds, I'll say 'done deal'.
However, that begs the question that what about the next day, when said load is all over the place?

I have a safe full of Savage rifles and most are Axis rifles with a few M12 in there. NONE of my rifles has ever fired a factory load, except .17 WSM....and we all know THAT story by now....

ron

I can't tell if you are making a statement or asking a question...

Darrell

If the numbers you get are meaningful to you, then what difference does it make whether it's a 5 shot group or a 50 shot group?

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Originally Posted by n4ue

A while back, I was pounced on because I stated that measuring bullet velocity and it's Standard Deviation, was not meaningful with 3 and 5 shot groups.
I recommended 32 shot groups. (been a long time since Stat Class in College)....
So, I did some research with Dr. Ken Oehler's topics concerning SD.... I have a Model 35P.

So, the good Dr. said trying to determine SD from 5 shot groups is "in the same league as trying to determine the accuracy with a 5 shot group". "Likewise, a SD number should be considered only as an INDICATION of uniformity".
He and other statisticians consider the acceptable sample size to be "20 rounds".

Now, I am the first to admit that every time I put a rifle/scope together, if it shoots a 5 round ragged hole at 50 or 100 yds, I'll say 'done deal'.
However, that begs the question that what about the next day, when said load is all over the place?

I have a safe full of Savage rifles and most are Axis rifles with a few M12 in there. NONE of my rifles has ever fired a factory load, except .17 WSM....and we all know THAT story by now....

ron

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I'm not sure what you're asking/saying either--although I THINK what your saying is that a statistical sampling field of only 3 to 5 shots is too small to derive any meaningful correlations of bullet accuracy?

Most of the time whether I do ladders or shot groups within the manufacturer's recommended charge weight ranges--I do end up with a sample field of between 20 to 30 shots--unless I'm mistaking what you're suggesting is that one should take 20 to 30 shots of the SAME charge weight to make an inference about the accuracy?

Seems to me that the probability of getting great results one day and lousy the next with all other equipment. environmental and shooter technique factors being constant is extremely remote--but an interesting question anyway.

If I shoot 5 shots in the same conditions with the affective variables all held as closely to the same value as possible I think it is reasonable to say that those shots are a bvalid and reliable indication of the accuracy of that load in that rifle. If I shoot 5 loads under these conditions and one is best with respect to least standard deviation from the centred POI then I am reasonably safe to say that that load is the best or the most accurate in that rifle at that time with respect to the other 4 loads tested. The problem is keeping all the variables at the same value for each shot hence the need to repeat the process a number of times for the closest results. I have used the 5 shot ladder method for years. I start with 5-7 loads based upon the start to max spread in powder. With all components kept consistent as possible, I repeat the process for the best 3-4 loads and then the best two and finally taking the best of these I adjust seating depth. It works for me but then I am only a sample of one and might be well away from under the centre of the bell curve. ;-)

Good topic BTW.

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Originally Posted by Hotolds442

If the numbers you get are meaningful to you, then what difference does it make whether it's a 5 shot group or a 50 shot group?

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Originally Posted by thermaler

Seems to me that the probability of getting great results one day and lousy the next with all other equipment. environmental and shooter technique factors being constant is extremely remote-

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Those right there, +1

If your numbers tell you something tangible & helpful, what someone else thinks is irrelevant.

Provided similar environmental conditions, you have a technique issue if results vary much/at all the next day.

It's a fact that the more shots you fire, the bigger the group will become up to a point. At that point you will be able to tell that the group size has stabilized; for me that is about 10 shots and from that you can calculate a useful standard deviation (SD). Statisticians like to use 20 as it is a rough rule-of-thumb regarding for normal, independent distributions (another statistical term). BUT WAIT! Take the same loads, rifle, range, etc and do this again tomorrow and you will not be surprised to find both the point of impact and the group size change; most likely due to the shooter. So you are better off shooting a smaller group size, like five shots across several different days, if you really want to characterize what is going on; and pool those results to predict how you are likely to shoot next time out. The same applies to manufacturing processes in that there is a certain amount of variability at a particular point in time, but also across different periods of time, and there is no rule regarding which one is greater. This is why the assumption for statistical independence which leads to the rule of 20 doesn't always work. For me as a shooter I expect more difference day-to-day than within any given day due to my performance vs the equipment.

CharlieNC, thanks for the reply. No, I wasn't asking a question, just trying to inspire a meaningful discussion. There are so many variables involved in this great hobby, it's amazing our guns shoot as well as they do! ha ha

I remember years ago when searching for a 'superior' brand of ammo for my TC 14" .22lr, I found myself paying more attention to the chronograph data instead of concentrating on group size. I stopped doing that and concentrated on group size since as you know, you can have a large SD and still shoot a good group.....

ron

roger that n4ue....shooters/reloaders have tons of questions and tons of answers.
hell...when Im doing load development, the freken voices inside my head start speaking out loud and i find myself asking......"hey who said that"?..lolololo
Its all good....................

I always try to warn people not to mistake me for someone who knows what they are talking about. It's just home-brewer's opinion.

Well, according to a book I'm reading "The American Rifle" 3 inches at 100 yards is accurate. Oh, and never use the smokeless .22 rimfire it will surely ruin your bore.

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Originally Posted by Joetinva

Well, according to a book I'm reading "The American Rifle" 3 inches at 100 yards is accurate. Oh, and never use the smokeless .22 rimfire it will surely ruin your bore.

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Exactly how old is this book?
That being said accuracy is directly proportional to the requirement of said firearm.

Darrell

Book is free for download from Google, written by none other than the famous Major Townsend Whelen circa 1910.

IMHO, a 5 shot group, from the bench, at 100 yards, that can be covered with a dime is accurate.
If you're a hunter and you can hit what you're shooting at, that's accurate enough.
But, if you're shooting more than a couple of shots and can't hit what you're shooting at, you better go home and do your homework.
As far as speed? If they're all going in the same hole, who cares how fast they're going?

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Originally Posted by Nor Cal Mikie

IMHO, a 5 shot group, from the bench, at 100 yards, that can be covered with a dime is accurate.
If you're a hunter and you can hit what you're shooting at, that's accurate enough.
But, if you're shooting more than a couple of shots and can't hit what you're shooting at, you better go home and do your homework.
As far as speed? If they're all going in the same hole, who cares how fast they're going?

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Mikie, just 'imagine' you came up with a load that gave great accuracy, but didn't know the speed. One hole? GREAT!

But if the speed is well below 'par', that's going to have an effect on trajectory. If you sight in the gun for all possible distance you might encounter, that OK. I prefer to know my bullet's velocity.

ron

Ron:
Depends on what you call "par". If the accuracy isn't there, it makes no difference how fast or slow the bullet is going.
Not a thing wrong with you wanting to know what speed you're getting.

I prefer to keep my loads at a mid range level for longer brass and barrel life. And with a mid range load, I have a little "wiggle room" when it comes to a temperture change.
You load for high speed at 70* and the accuracy is good. The temperature jumps to 100*, you can't hit squat with that load besides having blown primers and having to beat the bolt open. There's a fine line when building loads.
I'am limited to 100 yards for target shooting so knownig FPS of my rounds isn't a priority. I know I'am not overworking my brass. Got BR brass with 25 to 30 reloads and it's still going strong.
Now if I was in a position to be shooting a longer distance (for accuracy) I might be more concerned about the bullet drop over that distance.
On one of my rigs (6.8 SPC) I can shoot a clover leaf at 100 yards and without making any scope adjustments, with a little "holdover" I can hit steel out to 440+ yards. Got a couple of smaller calibers that will do the same. That's good enough for me.
Maybe not good enough for the next guy so we all have to do what we feel comfortable with.
My Chrony hasn't been out of the bag in years. Still works but not a priority for me.
Good thing we don't all think the same or it would be a "very dull world".
Mike.

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Originally Posted by n4ue

Mikie, just 'imagine' you came up with a load that gave great accuracy, but didn't know the speed. One hole? GREAT!

But if the speed is well below 'par', that's going to have an effect on trajectory. If you sight in the gun for all possible distance you might encounter, that OK. I prefer to know my bullet's velocity.

ron

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+1.......
"Tactically" speaking?.......the chronograph is a must.
(to me) theres nothing better than layzing a distance, dialing up your scope, reading the wind, sending the round and hitting your target 1st shot (preferably past 600)...IMO....that equates to shooting using "tactics"

fellow shooting buddy whom Ive shot with for 20 yrs or more is one hard headed retired marine. He absolutley refuses to accept any advise towards ballistic data tools to give him spot on hits at distance. He goes out and buys a 4'x8' sheet of sheet rock, puts it out at whatever distance he plans to work on, goes back to the firing line, sends the rds, drives back out to the his target, does his own figuring,goes back to the firing lines,,,and the routine starts all over again untill he finds what moa adj. he needs.
I tried in vain "ONE" time to have him give me certain measurements so I could use it as input for ballistic calculations but he cut my questions/conversation out right then and there. Ive never brought it up again, nor have I ever made any comments on his method. We,ve been shooting together on and off almost every sunday, for the past 20 yrs. Hes does his thing, and I do mine. I respect that, and leave it well enuff alone......dont wanna piss off a retired marine either.............:p