standard deviations vs accuracy

S

stagpanther

New member
I just finished testing some handloads (TAC 30 wildcat) using cutting edge Maximus bullets--and noticed the SD figures were excellent across many of the charges--several charge weights showed some of the smallest SD's I've ever had. Thing of it is, the smallest SD charges were not the most accurate, though I wasn't really shooting all that well at the time. My question is: is there generally a strong correlation between small SD's within a group and it's expected accuracy? I'm actually thinking it might be an indicator that I goofed up behind the trigger and should revisit the charge.
 
My understanding is that withing 100 - 500 yards its not relevant.

Somewhere out past there you get enough variation that by 800 t0 1000 is has an affect as the velocity being to make enough difference trajectory to be a larger and larger factor.

Conjecture on my part is the small SD is not in the barrel node and the barrel node rules at shorter ranges.

It may be a factor as low as you can get that the barrel is happy with as well. Knowing you might have a closer range node its fine at but at long ranges the good node is negated by the increasing factor of the velocity difference.

For long range it may well be a ballacne between the best node and good SD but not the best SD.
 
I rely pretty heavily on the 10 shot load development method and have found it to be pretty accurate at getting me in the ball park and that ball park correlates pretty closely with low SD’s. They may not be the absolutely most accurate load but they are always MOA or better and I’ll take a low SD, MOA load over a half MOA high SD load. My application is purely hunting so I’m happy with MOA but I want those low SD’s for longer ranges.
 
A consideration may be whether you'd pick a load that you are confident in it being more accurate , over a load that had lower standard deviation. If i had 2 loads that were essentially the same accuracy, may pick the one with the smallest standard deviation. How ever am of belief that most, including myself, do not shoot enough in a string (sample size) to have confidence the sd being generated is even valid.

It may be pretty easy to see a wildly inconsistent load, without the doing the calculation. Never really saw anyone using it, till it became an extra feature on chrono's.
 
I think that if you look at shooting as a set of variables, and if all the variables were exactly the same, you would get exactly the same results (fantasy, not possible on earth) then you would seek to "limit" each variable to minimum variance. SD is one variable.

With so many variables involved in shooting, having "stability" of some variables helps overall accuracy a great deal. Many variables add up to having chaotic SD, such as using brass with different case volume, neck tension, not precisely measuring charge weights, different primers, inconsistent shape of the same type of bullets, seating depth, to name some.

Certain powders provide lower SD if all the other variables that contribute are made consistent. To put it short, stick powders are the best for lower SD is my opinion. Ball powders are the worst. Worst being subjective only, its still acceptable...

guys on this board get under 10 SD with their best target loads, I am sure, and I would bet most of them, if not nearly all, are using stick powders to do it.
 
Stag I have found that a rounds accuracy depends mostly on the bullet and seating depth while the SD is more dependent on finding the proper charge and powder. I have a .223 load that is a tackdriver out to 300 but looks like a 00 buckshot pattern on a 600 yard target. I don't sweat SD on anything closer than 400.

You might try doing a Berger seating test on that load and bullet, it could turn into a sweet load
 
Stag I have found that a rounds accuracy depends mostly on the bullet and seating depth while the SD is more dependent on finding the proper charge and powder. I have a .223 load that is a tackdriver out to 300 but looks like a 00 buckshot pattern on a 600 yard target. I don't sweat SD on anything closer than 400.

You might try doing a Berger seating test on that load and bullet, it could turn into a sweet load
Click to expand...
It's also a high velocity load (for a stubby .308, that is)--which has the added bonus of being potentially a good hunting cartridge. The seating suggestion is a good one.
 
Last edited:
Another way to do it is you might load up 25 or 30 rounds and seat them into the lands. take a Lee handpress a seating die and you calipers. Once you get to the range seat three at magazine length and shoot them, if they are close together do a fourth, fifth etc and see if it groups. If the first three are 2 inches apart, bring the seating in another .010 and seat three more. Keep moving them in a bit at a time and you will start seeing a pattern. I have watched loads go from vertical to horizontal stringing just by tweaking seating depth .025. Also seen grouips tighten up a half MOA over .005 in seating depth change. What you want is when the group goes round goes round somewhere in the middle of the vertical to horizontal swing

If this was for a target gun I would start in the lands after doing a pressure check working my way up from .075 or .100 off. With a hunting load start at magazine length and make em shorter a bit at a time. I find most, not all, SMK's like .010 to .025, Nosler CC's seem to like around .025 to .035 but ya cant never tell. Like my Berger 80 gn load, .075 off and shoots bugholes. It's common knowledge on the net says Berger VLD's want to be jammed but don't tell my .223.

Oh and most of the time the decreased or increased case volume will not affect SD that much but it can. If so tweak the powder charge a bit if the load shoots bugholes with a preferred COL. Although that does not always work 100% of the time

here is one example I posted in another thread. Look at the way the group rotates as seating depth changes. Group 4 shot .6 MOA 10 shot groups at 300 the other day and I am taking it out to 600 on paper this week

https://thefiringline.com/forums/attachment.php?attachmentid=108758&d=1545151512
 
Last edited:
I may have forgotten to mention--this was in an AR15 based platform--which layers a couple of additional constraints on what you can do with the cartridge--though the TAC30--being based on the 6.8 spc case--is a bit more forgiving than some others.
 
I think the actual SD number would help to see . What is a large SD to you ? For me , under 20 is gtg but some would say that’s huge and Unacceptable. I rarely see single digit SD and might be why I’m willing to except the high teens . LOL
 
Last edited:
My findings...
Powder charge, bullet, seating depth affect accuracy at closer range. Along with this is tuning the action, on a bolt gun.

ES/SD are more closely related to powder itself, case prep, and loading proceedures.

I have seen bad ES/SD having an affect as close as 300 yards.
Case in point was my Stevens 200 in 7mm-08AI.
Using CFE223, Fed 210M primers, 139gr. Hornady SST.
Avg. 5 shot group at 100 yards just under 1". Groups at 300 yards, 4. 1/4".
10 shot string over chrono showed 42fps ES, 18 fps SD.

Switched to Alliant PP2000.
ES 7fps, SD 2 fps. 5 shot groups at 100 yards 0.78". Groups at 300 yards 2.13".
 
just like a hunting or tactical rifle, start them at mag length and work them in .005 at a time see what happens to the groups and monitor for pressure signs. I have 2 magazine fed bolt guns that I have developed loads for that way
 
I think the actual SD number would help to see . What is a large SD to you ? For me , under 20 is gtg but some would say that’s huge and Unacceptable. I rarely see single digit SD and might be why I’m willing to except the high teens .
Click to expand...
Three of the groups spread over a charge range of 1.2 grs were in the single digits. Admittedly, 4 shots per group is a very small statistical field sample, nonetheless, given 3 consecutive single-digit groups I consider a bit more than a coincidence.


attachment.php



This was using virgin SSA 6.8 spc brass which I resized, trimmed and "normalized" every case into the TAC 30. Used CFE BLK powder driving cutting edge 125 gr maximus monolithic bullets (first time I've tried this particular bullet, it's not an especially high BC bullet in this particular version--closer to a stubby flat base style 7.62 x 39 than a high BC spitzer/boattail really and feature a [my words] "drive band"). The bullets alone are about a buck a pop.:)
 

Attachments

  • Tac30.jpg
    Tac30.jpg
    32.9 KB · Views: 166
Last edited:
Sorry man but I have no idea what I'm looking at there :o . What was the SD of your best group ? Were your best group/s still in the teens ? If so , I'd say I'm gtg YMMV ;). You say you don't think you were shooting well ?? I've done limited tests showing a change in ES/SD depending on how I held the rifle . Holding hard and tight to the shoulder blew up my ES/SD . Just throwing that out there :)
 
Here's my build based on the ARP 18" tac30 chambering. It's a "point of pride" with me to cycle all my shots from the magazine--often just as much work needs to go into "accurizing" the AR action components as the cartridges themselves to get really good results. I typically use the cavity back open front magazine allowing me to go beyond the "normal" AR 15 COL length of 2.26. There are no tac30 dies--so it's a bit of a round about process getting there with the 30 Herrett die.


attachment.php
 

Attachments

  • IMG_1101.JPG
    IMG_1101.JPG
    148.3 KB · Views: 164
Last edited:
Sorry man but I have no idea what I'm looking at there . What was the SD of your best group ? Were your best group/s still in the teens ? If so , I'd say I'm gtg YMMV . You say you don't think you were shooting well ?? I've done limited tests showing a change in ES/SD depending on how I held the rifle . Holding hard and tight to the shoulder blew up my ES/SD . Just throwing that out there
Click to expand...
Well, you asked for the actual numbers--so there you go. :D the SD for this group was 5.66 fps. Im not here to pump the tac30 necessarily, but this is essentially a competitor to the 7.62 x 39 and 300 BO--the numbers it turns in are what I call "significantly impressive" for a "yet another AR15 wildcat." I was actually shooting out in the open in the freezing rain, so that might have been part of the "squirm" factor--but your point is well-taken on the hold. My "best groups" unfortunately featured 3 impacts close with one pretty wide flyer (which actually is pretty typical for me; I would change my forum name to "frequent flyer" if I could) so I often have to test the same loads more than once.
 
Last edited:
You must log in or register to reply here.
Back
Top