What is an acceptable Extreme Spread?

[jcampbellsmith]

Dennis

The Standard Deviation is the square root of the Variance.

http://en.wikipedia.org/wiki/Variance
http://en.wikipedia.org/wiki/Standard_deviation

Regards JCS

 

[DennisH]

Thanks, this info has helped tremendously

Dennis

 

[jlow]

Calculating SDev is pretty simple, here is the equation to calculate it:

http://standard-deviation.appspot.com/

Here is the very basic difference between ES and SDev with no fancy language. ES is just the difference between the fastest MV and the slowest MV thus the reason they call it “extreme spread”. Here is the problem with using ES, let’s just say you fired 1,000 rounds and chroned them and 999 rounds were exactly the same speed but one round was 200 fps faster, the ES would be 200 fps. Now let’s just say you fired another 1,000 rounds and they were all over the place in terms of MV but the difference between the fastest and slowest round was 200 fps, the group ES would then also be 200 fps. So you see, just knowing ES does not tell you how tight the MV of your fired rounds were just that the fastest was 200 fps faster than the slowest. Kind of useful but not very useful.

Now if you had SDev, that is different because that number gives you an idea of how tight the MVs were in the two groups because it takes into account the distribution of the numbers. So for the above example the SDev for the first group i.e. all the same speed except one would be a MUCH smaller number than the second example. That is why people find SDev much more useful.

 

[Nodak7mm]

The Standard Deviation is the square root of the Variance.

Thankfully for me, this is a "PFM" function of my chrony.... ;D

Rod

 

[Shynloco]

MVW,
I don't compete simply because my eyes aren't what they used to be regardless of which scope I use. But I have read articles written by top BR shooters as well as books written by the experts who are noted for their accomplishments in BR and long distance shooting. And, I read your loading regiment and the comments that followed. I might suggest, the comments by German Salazar are some of the best advice and ESPECIALLY the part about "uniform neck tension." Something you didn't mention is whether you anneal your brass/necks or not. Now I'm certain German not only turns his necks, but anneals them after every shooting so as to maintain a consistent neck tension. Something I also like to do that I learned from an ole timer is that he uses a Sinclair Mandrel Die in his reloading regiment. I guess the idea is much the same as using a Redding (or comparable) Competition Die. (To be clear, he used his Redding FL die, but removes the expander ball from it and allows the Mandrel Die to set his neck sizing with TWo consecutive pulls of the press handle). He swears by his method in following precision loading techniques. His finished .308 and 6mmbr rounds give him an excellent SD in velocity of no more than 5 -7 which is probably the best I've ever seen. I REALLY wouldn't have believed that was truly possibly had I not been sitting next to him and watching the read outs on his chrony this past week. Just something else to consider.

 

[amlevin]

But why are we all using the Standard Deviation where the variations are highly manipulated by the algorithm? Why not Mean Absolute Deviation? SD is more appropriate under ideal conditions but tends to distort when not.

I find that a load producing a low MAD that performs well can have a higher SD as well as ES.

In the end though, isn't it what shows up on the target on any given day all that matters? 8)

 

[jcampbellsmith]

But why are we all using the Standard Deviation where the variations are highly manipulated by the algorithm? Why not Mean Absolute Deviation?......

Good question. I found this handy link which is a variant on MAD.

http://easycalculation.com/statistics/mean-absolute-deviation.php

Thanks JCS

 

[Henryp]

At 1000 yards, extreme spread is all that matters. It defines the group size. I don't care what is inside it. Single digit only is acceptable. As I recall, every 6 fps is one inch vertical at 1000.

 

[jlow]

True, but if you are working on load development and trying to tighten MV spread, the SDev is a more useful number.

 

[GermanS1]

I should have added this link in my earlier posy, it is very useful.

http://riflemansjournal.blogspot.com/2010/07/statistics-for-rifle-shooters.html

"Statistics for Rifle Shooters" by Jerry Engelman

 

[effendude]

Thankfully for me, this is a "PFM" function of my chrony.... ;D

Rod
[/quote]

Ah, What Rod said!
Scott

 

[MVW]

Thanks to everyone for the very informative discussion. To capture many of the comments:

I loved the discussion and articles on statistics and as a physicist, the analysis appeals to me. I had been focusing on ES because that tells you if everything is inside the ring, but I have to admit that if you are driving the SD down, in general the load and loading technique must be getting better and will ultimately drive ES down. I am getting SD in the 6-9 range and ES in the 20-25 range, so I feel my technique is still a little lacking. I do turn the necks, but I have not tried annealing the cases (yet). I have lightened up the neck tension a bit and will see how that impacts the stats. I will try a different primer next. The nice thing is that I am using the same 13 pieces of brass in each test so that allows me to track results related to the brass.

I have been using SMK more for technique development, but I have Berger 185 bullets and 185 VLD's. Mostly I have been shooting at 300 yards, so I figure the less expensive bullets are probably reasonable in the interim.

I am impressed that folks are using scales with a precision of .002 GN. Mine is 0.02 GN and I can see the difference in each granule of powder, so that must be quite the scale.

FWIW - One comment on SD. There are biased and unbiased estimates, which are different. Not sure which one my Chrony uses, but I think it is different than the Excel calc. If folks are getting different answers that could be part of the reason.

Mike

 

[Leaddog]

The 175 SMK's are not a consistent bullet for 1000 yards. Try the Berger 185 LRBT's.

Thanks Charlie Watson, I guess I should tell that to my 175 SMK's next time I'm hitting X's at 1K. They apparently didn't get the message.

I've shot thousands of the 175 SMK's and they are a good bullet, but the Berger 185 LRBT does better in my 10 twist rifles. Interesting, none of the US rifle teams use the 175's anymore.

 

[MVW]

Mark and Others.

I followed up with TUBBS. It seems that they have discontinued their bullet comparator - for those that are intersted.

Mike

 

[jlow]

I’ve also looked at their site and you don’t see the bullet comparator anymore so that makes sense. However, it appears that they still list the BSC inserts for the various calibers, are those still available or are they gone too? Thanks!

 

[MVW]

Jlow, They mentioned that they still have the inserts in their reply. Mike

 

[MVW]

Jlow - it looks like Sinclair carries the bullet comparator. Mike

 

[jlow]

Thanks MVW for the info!

 

[nra-for-life]

hey mvw, this turned out to be a very informative thread.... please keep us posted if your are able to find out what lowers your spread im really interested to what you discover. also if you are interested in a very accurate scale check out the sartorious gd 503

http://www.youtube.com/watch?v=gfl9C0-5zYw

http://www.youtube.com/watch?v=byikXIrLlFM

 

[johara1]

MVW

Extreme Spread is not a useful measurement. All ES tells you is the difference between your fastest and slowest shot, it tells you nothing about the distribution of the intermediate velocities. The Standard Deviation is much more useful. Here's an interesting article on this.

http://riflemansjournal.blogspot.com/2010/02/reloading-velocity-decisions.html

I've got my Standard Deviation below 10 on some loads by focussing on neck tension. Firstly I have used bushing dies to full length resize (without the expander ball) and secondly I have neck turned to try and get consistent necks. Thirdly, I sometimes sort the necks as per the following article.

http://www.6mmbr.com/casenecktools.html

Good luck. JCS

Really? If your ES. is in low single digits SD. doesn't mean a thing..... jim