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Does case fill affect SD??


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been experimenting with Sport pistol powder in 40.  i loaded 4.9 grains with a 180 gallant at 1.18 for my 2011.  the velocity and SD were high.   i also loaded some at 1.13.5 to us in other gun and out of the same gun he SD was 10 or less.   at 1.13.5 it picked up about 75 FPS. the only difference is length.  did not touch the powder drop or anything other than turning the stem on my seating die.  


 the only thing i can think of is that the added volume of loading longer is affecting the SD numbers.  more space between the bullet and the powder.   so have you noticed this ?    i also did an experiment with RS comp which is a very filling powder and the FPS and SD were good.    

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Yup, COAL is absolutely something that you can vary for accuracy. I'm not an engineer to explain it in highly technical terms, but here's how I understand it.


As you reduce COAL you reduce case volume. In a reduced case volume the same charge will spike to a higher initial pressure at ignition. That is likely what gave you the higher velocity and better accuracy. Although you have to take care because higher pressure spike is also where you get primer deformation and case failure. If you had asked here first, "Should I take an established load and just shorten it by .045"?" you would get a chorus of, "heck no...do that stuff gradually and monitor for pressure signs!"


That being said, it sounds like your 2011 just wanted a bit hotter load for a bit better accuracy.


The other mechanic with COAL that your experiment went against conventional wisdom is to load the cartridge as long as you can to reduce the leap to the lands of the barrel. In a perfect world if I were you I'd lengthen the cartridge back out and increase the powder charge. That way you reduce the gap to the lands and you give the gun the slightly hotter load it seems to want.

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Bullseye shooters want the LSW to be just touching the lands for the best accuracy.  With jacketed bullets, that is a no no.  It will spike pressures.  If your 2011 will reliably feed 1.135", load that for everything.  My CZ TS liked 1.126" best and both 2011 would feed that length.  So that is what I loaded.  You also need less powder for major.


Powder needs a certain pressure to burn completely.  If it doesn't get that pressure, large SDs are the result.  As you inch up the pressure curve your SDs get better.  Also be aware that SDs go up and down as you move up the ladder.  For example, at 3.4gr of my powder under a LSWC returns an SD 04.97.  Bump to 3.6 and the SD drops to 11.85.  Bump to 3.7 and SDs are 9.42.


I'll assume you are shooting Limited.  None of this matters for accuracy in run gun.  The difference are too small, and the A zone is big.

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  • 2 months later...
  • 4 weeks later...
What does SD stand for? Also, a brief explanation of why it's important would be great.   ..bear with me as I'm new to reloading 



SD is the standard deviation between the velocities of your loaded rounds. A larger SD would indicate the difference in the velocity spread is larger. The idea is that 2 bullets moving at exactly the same speed will hit the target at the same place. Variations in the velocity would create differences in that point of impact. That of course is assuming that your sight picture is identical. 

I personally don't get all worked up over SD. This tends to matter a lot more for precision rifle shooting and maybe bullseye, but no so much for flinging lead short distances at a pretty big square.


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On 1/19/2020 at 9:13 AM, doane said:

What does SD stand for? Also, a brief explanation of why it's important would be great.


..bear with me as I'm new to reloading


I disagree with the analysis of SD above.  A load with a huge SD will still all land in the A zone at 25 yards.  SD basically stands for consistency.  The lower your SD, the more confidence you have your loads will chrono major at a match.  If the SD is large, you have to go up in power factor to make sure each round makes major.  Then you put up with more muzzle rise and more recoil.


Here are a couple of examples:


My first Limited load had an SD of 4.84 and a PF of 172.  The slowest round went 953fps.  The fastest was 966fps.  So the slowest load had a PF of 171, well above the 165 PF floor.  In fact, I later reduced the load to a 168 PF and the slowest round still made major.  So I reduced recoil and muzzle rise and had a more comfortable load to shoot.


One load I tried had an SD of 17.97 and a 170 PF.  One of those rounds did not make major.  So there was a 10% chance that one of the three rounds chosen for chrono would not make major.  Since several other rounds barely made it, there would be a real risk of being reduced to minor.  If I decided to use this load, I would have to bump it up to 172 PF to be sure of making major. 


With higher SDs you have to bump it up even more.  Personally, I'm happy with single digit SDs.  My Open load had an SD of 5.90 for a 15 shot string.  No matter how many times I try it the results are the same.  169 PF.  Minimum PF is 168.2.  With my last Open load I had to be at 172 PF to be sure of chronoing major.

Edited by zzt
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