Does SD really matter? Why is mine so high?

scott_r

scott_r

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I have a .204 Ruger load I worked up for coyotes this fall thats a solid 1/2" but the SD is 40...velocity seems to be all over the map but I get some really good groups. My Max distance shots will be 400 yards...no long range shots and am wondering if a high SD really matters? Im not sure why its so high either. Im using the same brass lot, trimmed to the same length. Power is measured precise...CBTO measurements are uniform. Using Berger 35 gr match/varmint.

Cheers!!
 
If it shoots good groups at 400, then kind of who cares with the numbers. Real world groups matter more then those numbers to me. :)

If it was a target rifle I would worry alittle more.
 
RojamB21 said:
In simple terms,

SD, Sectional density is how strong, dense the bullet is built.

BC, Ballistic coefficient is how good, stable your bullet fly in air.
Click to expand...

I think by SD he is referring to standard deviation.

SD of 40 may feel a bit high for a long range rig, but you would also have to be positive that the chrono you are using is accurate. If your groups are acceptable to you at the ranges you intend to shoot, I wouldn't beat yourself up over 40fps. Try keeping your Ammo making consistent and your shooting consistent and only try solving problems that you actually see on the target.
 
What does the SD look like in comparison to average velocity? 40 of 1000 fast is 4%. 40 of 3000 is only 1.3% variation. SD is a means of more accurately averaging wide spreads in datum used to calculate an average and shows how much the final average can fluctuate.
 
scott_r said:
Im using a cheap one...Im thinking the more I read about it that it may be the cause. Its a Caldwell.

For you vets out there..how should I be setting my chrony up and in what light conditions. Im about ready to put a bullet through mine.

Cheers!!
Click to expand...

I am no vet Scott....... But I trust results on paper way more than I trust a crony....... If you have slid groups out to 400 you have a solid load there....... My personal limit is 300....... And will be working at 400 based on an upcoming hunt.......

That being said, your results sound excellent with your load........ I would stick with it......
 
scott_r said:
Im using a cheap one...Im thinking the more I read about it that it may be the cause. Its a Caldwell.

For you vets out there..how should I be setting my chrony up and in what light conditions. Im about ready to put a bullet through mine.

Cheers!!
Click to expand...

Results from the cheaper chronys should always be taken with a grain of salt.
I know with the beta chrony there are a couple of mods you csn do to improve its accuracy. Not sure about the caldwell.
If you have nice tight vertical at 400m, I would roll with it. How likely are you to push a 204 past that anyway?
 
What's your muzzle velocity?
An SD of 40 on something at 400 fps is a bigger deal than an SD of 40 with an average of 3600fps.
Keep in mind that standard deviation is a measure of how high a peak you will get on your curve, aka bell curve. A smaller SD will give you a taller and narrower bell, putting more of your samples closer to the average.
 
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RojamB21 said:
In simple terms,

SD, Sectional density is how strong, dense the bullet is built.

BC, Ballistic coefficient is how good, stable your bullet fly in air.
Click to expand...

Wrong sectional density is the ratio between the bullet's weight and its diameter (or more precisely it's cross-sectional area, which you can find using pi*r^2 or pi*d^2/4). It has nothing to do with bullet construction.
 
If the rifle is shooting to your satisfaction, at the distances at which you will be using it, don't worry about SD.
Velocity variation will make a difference at long range, vertical stringing will be observed, so a small SD is desirable then.
 
My 223 load shoots about .75 all the out to 300 yards. the farthest shoot I've made on a coyote is 340. I can't even remember what the ES and SD is for that load because to me it doesn't matter and likewise your SD of 40 shouldn't matter for you. If your were shooting out to 600 or so yards it would start affecting your groups vertically.

I'm also not a vet, but I shoot a fair bit. I usually use my crony info as a starting point to start shooting out to longer distances. Running my numbers through a ballistics calculator to figure out holdover using a mill dot scope. It has worked out very well for me, but I wouldn't make any assumptions until it is proved on paper.

I have an Alpha Crony and the one thing I noticed is a very bright day tends to give me high readings. Cloudy days are slower readings and more consistent.


It sounds like you have a great load there.

George
 
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Hey Scott

I was just wondering, how much of a jump is your rounds because if they are real close I think that could make a good variable jump in fps because the rounds could be off by a few thou.
 
A standard deviation of 40 is quite high - Litz talks about an SD of 10 being possible with careful handloads, and 15-20 bring typical of factory ammo.

3 possible things to check:
1. Sources of chrono error - alignment especially as small angle errors can be an issue.
2. Powder charge consistency - measure to 0.1 grain if possible
3. Neck tension for consistency of release

Maybe try a friends chrono, and if that's the source, then put a bullet through yours (usually just a matter of time anyway)...

Hope this helps.
 
titch-- said:
Hey Scott

I was just wondering, how much of a jump is your rounds because if they are real close I think that could make a good variable jump in fps because the rounds could be off by a few thou.
Click to expand...

My .204 is a Tikka and due to the short mag length I can't come anywhere near the lands on the rifle. It's sitting a few thousands of an inch longer then saami spec.
 
Real world results is everything.


Laugh2 It would be simpler when one explains the SD the OP is referring to. One sounds a lot smarter when they are on topic.
RojamB21 said:
In simple terms,

SD, Sectional density is how strong, dense the bullet is built.

BC, Ballistic coefficient is how good, stable your bullet fly in air.
Click to expand...
 
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