Standard Deviation (SD)

[LynnJr]

Metal Dog
I think you misunderstood BartB who ever he is? If you meant Bart S I think you might have misinterpreted what he was telling you.
Your hold on the gun doesn't affect velocity for all practical purposes.
It definitely affects vertical stringing and your group but not your velocity.
Slow shots usually mean the barrel isn't fouled and fast shots usually mean you let your rounds heat up in the chamber too long.

[Thoughts]

The lower SD will, however, result in more "lucky" small groups over a long period of time.

Luck has nothing to do with it. A small SD is tied by definition to tighter groups, all else equal.

As it is the worst shot that defines your accuracy that is more valuable to me.

That's a perfectly reasonable choice.

Really, SD doesn't mean beans unless you are working with large result sets that can be normalized.

That's simply not true, and I have no idea what you mean by "normalized" in this context. The power of statistics is that you can use a small sample size to determine the performance of, say, a batch of ammunition.

Stats guys use standard deviation, and math guys use the square root of the variance.

The standard deviation is the square root of the variance. That's the definition.

[Metal God]

Bart B is Bart Bobbitt a Service rifle shooter mostly I believe . Over the years we have talked through forums and PM . I believe my test was exactly what he was talking about . We literately talked before the string and after , so I believe we were on the same page . It was awhile ago though but my memory is not that bad "yet" . I'd link the thread but I can't find it

As for My ES/SD difference in my test . Everything was the same except the hold on the rifle . When I say everything I actually mean it . The test was strictly to test the velocity's and ES/SD , I did not even try to shoot a group . I weighed each charge with a Redding beam scale with check weights and using tweezers to throw the last couple pieces of powder in the pan to weigh perfect . I had my inferred thermometer with me to insure the strings were started with the same barrel temp . I timed the first string shots and shot the second string in the same intervals . I made sure all the rounds were handled in the same way so powder was not up against the bullet on one shot and back against the flash hole on the next . I feel I did all I could to insure an accurate test .

I really do try to only post things I'm sure are accurate . If I'm not sure I use words/terms like "It's my understanding" or " I believe" . In this case I stand by my test and until someone else proves it wrong with a test of there own I will keep posting it as fact and not theory .

People should keep in mind that my test above was with a stock-ish rifle . "I believe" it represents a more stock factory rifle result then lets say a rifle that is 20lbs+ and is why I mentioned bench resters likely never see this issue . A heavy rifle will likely give you better ES/SD results just by the weight of the rifle alone and how little recoil there will be compared to , lets say a 8lb rifle . . Most guys will hold that 8lb rifle much tighter then a big heavy one effecting there ES/SD more . But this last part is an opinion that has not been tested .

[LynnJr]

Metal Dog
I am not disputing your results just what caused them. If your hold caused them it would be repeatable at any point in the string and you haven't posted that you tested it and got the same results. A 50 fps variation is huge.

Thoughts
Standard Deviation does not determine your groups size Extreme Spread does.

For all you guys thinking standard deviation is the answer your kidding yourselfs.

[ar15barrels]

I am disputing your results just what caused them.

[Thoughts]

Thoughts
Standard Deviation does not determine your groups size Extreme Spread does.

Any reasoning you can share? The ES of one group is that group's "size", but the SD is a better predictor of future groupings and the total dispersement.

[bsumoba]

Rifle & load
308 Savage model 10 with heavy 24" barrel shot from a bench with front and rear bags
40.7gr IMR 4895 / 175gr smk / WLR primer / WCC-06 cases

Keep in mind all these rounds were loaded at the same time with all the same lot components and all were handle from my reloading bench to the rifle in the same manner

First string was shot with the rifle pulled in hard and tight to my shoulder for each shot .
Results = 2517 / 2582 / 2568 / 2552 / 2568 / ES-65.52 / SD-28.60

20min later with a cooled barrel I shot the second 5 shot string . This time I barely even touched the rifle and let it freely recoil for the most part . I did have to keep in control of the rifle but in comparison to the hard tight hold it was like not even touching the rifle .

Results = 2551 / 2555 / 2559 / 2552 / 2565 / ES-14.50 / SD-5.74

Remember same load , same rifle , same conditions . Only difference , how I held the rifle .

As you can see the first shot on the hard ,tight hold was slower then the rest . More then the 30fps I had been getting but I also had never held the rifle that tight before .

I believe this test and my subsequent test on other loads shows there is a direct correlation to how you hold the rifle and what your ES/SD will be .

I respectfully disagree with your results as Lynn does.

Now, look at it this way:

Take out the 2517 shot, and now your SD goes down to 12.2 and your ES changes to 30 for the 1st group. Sounds like you had a "bad" round that wasn't prepped the same way. This tells me you need to work on the consistency of your rounds. You may think you did everything the same, but maybe you had a piece of bass case that had significantly different case volume than the rest. Maybe your neck tension was different than the rest. Maybe your CBTO was off because the bullet was just made differently (much different base to ogive than the other bullets in the sample). There are a lot of variables that the normal handloader doesn't pay attention to.

This is a perfect example of how one should review their ES rather than the SD. Control the ES and you control the SD...it's so simple. That 2517 shot would have most likely resulted in a flyer or a 9, or even 8 or 7 in a match for f-class. Sometimes you need to look for "flyer" rounds.

One thing I will be doing soon, if somehow I can do it is to shoot at a match and if a shot results in a huge vertical dispersion from the rest of the shots and no one else lost a shot in the same manner (indicating that it was most likely not a condition that generated that shot deviation), then set that round aside and further evaluate that round. Maybe that case has something very different about it. Just another thing to test I guess.

Now....

Take the 2517 and the 2582, which I would call your two "flyers" in your 10 total shots and combine the two groups and you get:

2568,2552,2568,2551,2555,2559,2552,2565

That is a SD of 7 and ES of 17. This would be acceptable for most precision shooting, assuming you did not have those 2 shots.

So, what does that tell us? It is most likely not the hold of the gun. It is the fact that you had 2 shots out of 10 that went outside the group and like myself and Lynn and a few other guys mentioned, would most likely result in a flyer or bad score for that shot. Those two rounds were just not noticeably different than the rest when you were making those rounds.


To dispute your theory, I have done testing on different holds on my rifle as well. I have shot with hard cheek welds, light cheek welds, hard grip, no grip, rifle positioned just touching the front stop on the rest and me loading the rifle onto the front stop. I never saw a deviation in my velocities like you saw. The only deviation I see is that as I put my rounds down my barrel, velocities will increase slightly around round 50-200 and then it stabilizes for some time. Then, it will slowly start decreasing as the barrel wears.

[LynnJr]

Any reasoning you can share? The ES of one group is that group's "size", but the SD is a better predictor of future groupings and the total dispersement.

If you ever get the chance to shoot a truly accurate rifle the first thing you will discover is that your biggest groups have the most velocity spread.
And Standard Deviation is not a better predictor of group size. It doesn't predict anything at all just gives us a hypothetical assumption. Extreme Spread is real and SD is not.
We just held a PacRat Invitational in NorCal and had about 11 different shooters all shoot sub quarter moa groups with my 300 WSM. The extreme spread is around 6 fps.
I will try and post a picture. In the picture the shooters face never makes contact with the stock and his shoulder should be about 1/4 inch behind the recoil pad just to catch the rifle. Notice it sits on tracking rails and it doesn't need any hold from the Shooter.

[Metal God]

Take out the 2517 shot, and now your SD goes down to 12.2 and your ES changes to 30 for the 1st group. Sounds like you had a "bad" round that wasn't prepped the same way. This tells me you need to work on the consistency of your rounds.

This would be incorrect , Why ?? Because I was having that same slower first shot issue in my strings for over 300rds 5 shots at a time over multiple different loads . I always thought it was my bore and a heat thing . FWIW those shots were 105 though 115 maybe 115 through 125 after cleaning . I worked on this issue for awhile ( couple months ) and that first shot coming up slower happen way to consistently for it to be case prep . It has never come up since I started holding my rifle differently .

I understand you guys want to use theory on why it may have happened . How ever you all seem to be real good reloaders . It only takes 10 shots for you to run the same test your self . Give me an honest test and report back your results . At that point we can debate each others results . You can't use 25lb rifles though , 10 12 lbs max . One string have a death grip the other let the rifle freely recoil while staying in control .

As far as ES vs SD . I never meant to say SD is better predictor of likely group size . I use SD to tell me what the likely hood each shot will be . This helps me more because I don't always know the distance to target . I'm looking for first round cold bore hits and is why I was working through that first shot being slower issue . If I have a shot I think is 565yrds I prefer using the SD because it gives me the "likely" speed my bullet will be traveling rather then the the two extremes it "may" travel . If I have a MOA target I'm trying to hit at that distance . Doping my rifle to either end of the extremes is not going to result in a likely hit . Especially if that extreme is the 2517 velocity .

This may explain why we all seem to talk past each other . We are shooting and testing for two completely different types of shooting .

oh and Lynn that's a real nice looking rifle !

[bsumoba]

Any reasoning you can share? The ES of one group is that group's "size", but the SD is a better predictor of future groupings and the total dispersement.

groups taken over 2 different days and one is at 100 yards and one at 200 yards.

ES was 14 on the first day and ES was 12 on the second day over 5 shots (different group than what was shot below since I use a magneto) .

The 200 yard, 5 shot group is 0.57" edge-to-edge. This is a 6 dasher.

I don't track SD.........

[Thoughts]

groups taken over 2 different days and one is at 100 yards and one at 200 yards.

ES was 14 on the first day and ES was 12 on the second day over 5 shots (different group than what was shot below since I use a magneto) .

The 200 yard, 5 shot group is 0.57" edge-to-edge. This is a 6 dasher.

I am not sure what point you're trying to make.

In the case of a very bad round, a control chart could find that.

[bsumoba]

I am not sure what point you're trying to make.

In the case of a very bad round, a control chart could find that.

Low ES coupled with a tuned rifle will shoot better groups typically.

Put it this way...I would rather know what my extreme's are rather than what my extreme's could be.

[LynnJr]

When you read about shooters saying this is my group with 4 good shots and a flyer those shooters tend to use standard deviation.

When you read about tiny groups without flyers those shooters seem to ignore standard deviation and use extreme spread exclusively.

[ar15barrels]

Any reasoning you can share? The ES of one group is that group's "size", but the SD is a better predictor of future groupings and the total dispersement.

We are talking about chronograph velocity data.
Chronograph velocities and spreads do NOT determine group sizes on target.
Group sizes are determined by measuring bullet hole relative locations on target.

[ar15barrels]

Now, look at it this way:

Take out the 2517 shot, and now your SD goes down to 12.2 and your ES changes to 30 for the 1st group. Sounds like you had a "bad" round that wasn't prepped the same way.

Also could have been the first shot of the day through a cold dirty barrel.
I have a theory on cold bore differential that is related to oxidization of the bore contents over the days/weeks since the last shot was fired.
Once you "wipe" the bore with a bullet and powder charge through the barrel, that oxidization gets nornalized again for the day's shooting session.

Firing a round the day before a match seems to really normalize my cold bore shots.
I now make a habit of stopping somewhere on the side of the road whenever I travel and letting loose a few rounds in conditions closer to my match location to check zero and gather hard dope whenever I can.