What do SD #s REALLY mean??

[sierracharlie338]

Here is the dilemma I have. I load for my 223 bolt rifle using Lapua Brass, 69 SMKs and federal SR primers. I am getting very accurate loads out of this rifle. I have shot it out to 500 and it maintains around 1/2 moa (yes a 2.5 inch group at 500). I recently got a Magnetospeed Chronograph and put it on this load as i was curious what the speed might be since accuracy was so good. The SD showed to be around 23 after 14 rounds. While speed was around 300 FPS slower that it should be with that bullet (2640 fps).

To give you a background on my loading procedures (which I have found to be labeled as "anal" after a recent thread) they are as follows. FL size 223 brass, throw a charge with Hornady electronic powder dispenser that is short of target weight, then I trickle up to final weight. Seat the bullet and put it in the box. I am not sure how I could be more specific with my loading procedures without wanting to eat one of those rounds.

I guess what is making me scratch my head the most is you see everyone talking about "look how low my SD numbers are"... Best I can tell, low SD numbers don't necessarily mean shit if my rifle shoots as good as it does with an SD in the 20's. Any thoughts or suggestions are welcome.

This is a pic of a 5 round group I shot at 100 off a bipod and bag when I was using the Magnetospeed today. 1 inch circle

 

[Milo 2.5]

SD means deviation from the average. If you were to talk to benchrest shooters who win, a good SD number is something they do not concern themselves with. The target is everything.

 

[spife7980]

Standard Deviation is a statistical measure of the spread and how condensed variances are and it means a decent bit at extended distances. If your ammo isnt consistent then neither will your hits be way out there.

1 standard deviation is the green, 2 standard deviations extends out to the yellow and red is 3 standard deviations. If you have a standard deviation of 10 (the green area of the chart only) that means that 68.2% of your shots fall inside of 20 fps difference from your average (+10, -10). Now to figure out how this would effect your shooting at say 700 yards take your ballistic calculator and enter your info exactly as you record it. Im going to use my 70gr rdf stuff that Im working on currently for figures. At 700 yards my 70 gr rdf is 21.7 moa to dial at my recorded 2617 fps muzzle velocity. If I drop that down to 2607 (-10) its 21.9 moa and up at 2627 (+10) its 21.5 moa. So for my particular load if I were to have a SD of 10 then I would have a group size that is going to be mathematically limited to being .4 moa larger than it would be in a perfect world with exactly the same muzzle velocity. If I were to translate that to a linear unit of measurement than my group would be limited to being a minimum of 1.6755" just due to differences in shot to shot velocities. And this isnt factoring those outlier shots that fall outside of the 68.2%. 30% of your shots that will be either higher or lower than the 1.6755" we just calculated and thats with a bullet that has a great BC, use a crappy flat nose 150 grain 30-30 bullet and the difference will be exaggerated much further. At 1k for my 223 its 43 moa of hold on the low end of velocity and 42.2 on the high. That 8" of minimum difference just due to velocity inconsistencies for roughly 2/3 of your shots.

To use your SD of 20 at 700 yards its 22.1-21.3moa of hold for .9moa/6.6" of velocity related dispersion and at 1000 its 43.4-41.8 for 1.6moa/16.75" of velocity related dispersion.
You can see how cutting the SD in half improves your group roughly 2 fold in the vertical aspect in this example. Now if youre shooting a target 24" tall then youre hits will still fall on target. If youre shooting a 10" plate many of them would not be hits.






So how much does it play into real world scenarios? It depends on how far youre shooting and how poor your SDs are. Obviously my example isnt extreme and shows that while the group will be larger than I would like it probably wont equal a miss if Im aimed at the center of the target. Take that out to 1k and youll definitely see a much larger effect on target. So its kind of a sliding scale between what your numbers are and what youre attempting to use them on. for under 300 yards no one cares about that. The further you go the more varied your results will be.

At 100 velocity hasnt had a chance to really make a difference, once you get it out there further away youll notice much more of the effect. Youll shoot three times with a perfect shot on each attempt and you get a hit, shoot over the second and shoot under the third.

Read this for a bit more statistics http://precisionrifleblog.com/2015/0...oes-sd-matter/

 

[Sheldon N]

SD means deviation from the average. If you were to talk to benchrest shooters who win, a good SD number is something they do not concern themselves with. The target is everything.

Of course to those guys "good" SD is 3 and "bad" SD is maybe 7-8.

SD is just a measurement that is an approximation for what we really care about... vertical dispersion at distance. If you can skip the measurement and just test the end result... group size at distance... that's preferable.

If you want to run hypothetical scenarios JBM Ballistics is good for that. Run your 1000 yard drop data at your average fps. Add 20 fps and re-run it, how much higher will it impact? How much lower at 20 fps less?

 

[sierracharlie338]

Interesting info here guys. Sadly it doesn't make me feel any better about it haha.

Thanks for all the responses. I suppose for my heavier rounds I will take this into consideration but a 223 that will be kept inside 500 probably not. I will however be doing some more research on it.







Sierracharlie338....

 

[spife7980]

Yeah, my old 69cc load was an ad of ~15 and I thought it shot lights out out to that distance beyond which my 16" just doesn't have the ass to make it as well. Working the 70 rdfs now and the charts show a pretty drastic improvement. I'm excited to try them. But at long range stuff it makes a huge difference.

There is some school of thought that thinks if you get the node right you can time the barrel so that with a high sd the slower bullets are on the high end of the barrel whip arc that will throw them relatively higher than the higher velocity rounds exit at a lower point creating a situation where they converge more accurately at a distance. Now while I assume that's theoretically possible I figure it's much more attainable to shoot them at a consistent repeatable velocity that would work better across all ranges rather than tryin to game the system to a given convergence range.

 

[morganlamprecht]

500 yds doesnt start to be a problem...it takes a 30 fps change in my 223 load to even change my dope 0.1 mil

the "300 fps slower than it should be" could be an issue as well...slower as in the velocity given didnt match your dope? could be an issue with how you have the MS mounted causing incorrect readings...if talkin about 300 fps slower than book data, book velocity data doesnt mean anything most of the time

 

[sierracharlie338]

500 yds doesnt start to be a problem...it takes a 30 fps change in my 223 load to even change my dope 0.1 mil

the "300 fps slower than it should be" could be an issue as well...slower as in the velocity given didnt match your dope? could be an issue with how you have the MS mounted causing incorrect readings...if talkin about 300 fps slower than book data, book velocity data doesnt mean anything most of the time

When I said that I was mainly referring to where I would like it to be vs where it actually is. However since you brought that up, the book does say it should be closer to 2900 than 2640. I may tinker with it some and see if I can find the next sweet spot otherwise I probably will just leave it as it will be mostly for irradiating varmints and an occasional deer inside of 300 yards.

 

[spife7980]

How long is your barrel? Because my 16" 26xx is in the ballpark of my experience.

Does your dope match your calculator? Aka does your velocity of 2900 line up with the calculator signaling that your chrono is defective or does it actually line up and you've just not realized it until now?

 

[sierracharlie338]

How long is your barrel? Because my 16" 26xx is in the ballpark of my experience.

Does your dope match your calculator? Aka does your velocity of 2900 line up with the calculator signaling that your chrono is defective or does it actually line up and you've just not realized it until now?

Its a factory Rem Tac SPS barrel at 20 inches I think?? I have not even checked my dope versus my drop data. I will have to try that when I have some time to go back and find it. I sure the hell hope that my MS is not defective. It was the first time I used it haha

 

[spife7980]

At 20" that velocity sounds about right and you're not being too hard in the brass, trust the chrono. For 2900 I would expect a 26" barrel and 55 grain bullets. With your heavier bullets on a mid length barrel you're not going to ever get 2900.

 

[TonyTheTiger]

Sounds pretty slow to me. I've chrono'd my 16" and 22" gas guns, and the 16" will easily do 29xxfps with 68/69gr bullets and above 2800fps with 75/77gr Bullets. The accuracy nodes are about 50-80fps below those speeds. The 22" runs everything around 150fps faster for both max charge and accuracy sweet spots.

 

[spife7980]

What ammo/powder is getting you 2900 out of a 16" barrel gas gun?

 

[TonyTheTiger]

8208 and Tac in LC brass with CCI450'S. Primers started to show pressure and it's hard on the brass because my gas is tuned for my close range bullet hosing loads but it is possible. Like I said though, best accuracy is a couple notches below those max charges.

 

[morganlamprecht]

my 16" wont easily do above 2800 with 77s...had to fill the case up with like 24.5 grains of 8208 to even get to 2650...was already seeing ejector marks on brand new LC brass

 

[TonyTheTiger]

I load all my heavy bullets at 2.31" in ASC mags, it helps a little. I definitely haven't seen any ejector swipes though.

 

[Smithcollector]

I've found a (relatively) lot of variation in the ogive to base measurement on SMK's, more so on older bullets than the newer ones. Difference ogive to base measurements means the seating stem of your die will touch the bullets at different points, which will change your seating depth, which could cause some of your velocity variations.

 

[sierracharlie338]

I've found a (relatively) lot of variation in the ogive to base measurement on SMK's, more so on older bullets than the newer ones. Difference ogive to base measurements means the seating stem of your die will touch the bullets at different points, which will change your seating depth, which could cause some of your velocity variations.

This is no shit! I had one lot of 175 SMK in the 100 round box and one lot in the 500 round box. I was so surprised at the variation in measurement I even emailed Sierra and was basically told, yeah that can happen..... WTAF??