Standard deviation

[Red Ryder]

I loaded up a bunch of IMR SR 7625 4.6 grains with 124 grain MG CMJs loaded out to 1.155 will all Blazer and Speer brass. I chronoed 20 round strings at a power factor of 131.6 consistently, which is conistent with a friend's data. My standard deviation was 19. Is there anything I can do to tighten up the Standard deviation?

[kevin c]

Do they shoot well, by which I mean, do they feed reliably and are accurate? If so, maybe it's not worth worrying about, except the already very small chance of stringing together a few low rounds and not making the PF threshold (somebody with better math skills could probably give you the odds).

Lowering the SD takes a very high degree of consistency in everything that goes into the load. That includes all the individual components as well as the loading technique. Soooo, if you wanted to, you could use single head stamp same lot new brass, or same lot once fired all shot through the same gun, weigh and segregate single lot bullets by weight, use single lot primers, and load at a slow but steady and sustainable rate on a single stage press to eliminate the OAL variation that comes from using a multiposition shell plate. You could also change to a powder known to meter very consistently, like the tubular Viht types.

Having done just about all of the above, I'm not sure it's worth it. With a 6"x11" A zone, the need for super accurate ammo isn't there. That leave's PF, and raising the powder charge to a bit higher takes care of that, even if the SD doesn't shrink (though it might, since my impression is that many powders burn more consistently at higher rather than lower charges). I do 135 PF; it's as accurate as I can use, my Glocks like it, and I never worry about making declared PF.

Edited April 8, 2011 by kevin c

[P.E. Kelley]

I loaded up a bunch of IMR SR 7625 4.6 grains with 124 grain MG CMJs loaded out to 1.155 will all Blazer and Speer brass. I chronoed 20 round strings at a power factor of 131.6 consistently, which is conistent with a friend's data. My standard deviation was 19. Is there anything I can do to tighten up the Standard deviation?

Nothing needed! An SD of 19 is just fine, you should be happy.

Don't get lost the minuscule.

Load a bunch and shoot.

Patrick

Edited April 8, 2011 by P.E. Kelley

[Red Ryder]

Do they shoot well, by which I mean, do they feed reliably and are accurate? If so, maybe it's not worth worrying about, except the already very small chance of stringing together a few low rounds and not making the PF threshold (somebody with better math skills could probably give you the odds).

Lowering the SD takes a very high degree of consistency in everything that goes into the load. That includes all the individual components as well as the loading technique. Soooo, if you wanted to, you could use single head stamp same lot new brass, or same lot once fired all shot through the same gun, weigh and segregate single lot bullets by weight, use single lot primers, and load at a slow but steady and sustainable rate on a single stage press to eliminate the OAL variation that comes from using a multiposition shell plate. You could also change to a powder known to meter very consistently, like the tubular Viht types.

Having done just about all of the above, I'm not sure it's worth it. With a 6"x11" A zone, the need for super accurate ammo isn't there. That leave's PF, and raising the powder charge to a bit higher takes care of that, even if the SD doesn't shrink (though it might, since my impression is that many powders burn more consistently at higher rather than lower charges). I do 135 PF; it's as accurate as I can use, my Glocks like it, and I never worry about making declared PF.

Reliable Yes. Accurate Yes. I can ding 6 inch plates at 25+ yards. I do need to bench rest shoot some paper to say for 100% certainty. I do like the way N320 meters. I am working on my third load for 9mm. I just REALLY like the soft feeling of 7625 compared to Titegroup and N320. I like the N320 too, but not as much as the 7625.

[Blank Zero]

So sorry for a noob question, but what does Standard Deviation do? Or represent in reloading?

[sirveyr]

So sorry for a noob question, but what does Standard Deviation do? Or represent in reloading?

It shows how much variation there is from the mean value. A low standard deviation indicates that the data points tend to be very close to the mean, whereas high standard deviation indicates that the data are spread out over a larger range.

[RH45]

Do they shoot well, by which I mean, do they feed reliably and are accurate? If so, maybe it's not worth worrying about, except the already very small chance of stringing together a few low rounds and not making the PF threshold (somebody with better math skills could probably give you the odds).

Lowering the SD takes a very high degree of consistency in everything that goes into the load. That includes all the individual components as well as the loading technique. Soooo, if you wanted to, you could use single head stamp same lot new brass, or same lot once fired all shot through the same gun, weigh and segregate single lot bullets by weight, use single lot primers, and load at a slow but steady and sustainable rate on a single stage press to eliminate the OAL variation that comes from using a multiposition shell plate. You could also change to a powder known to meter very consistently, like the tubular Viht types.

Having done just about all of the above, I'm not sure it's worth it. With a 6"x11" A zone, the need for super accurate ammo isn't there. That leave's PF, and raising the powder charge to a bit higher takes care of that, even if the SD doesn't shrink (though it might, since my impression is that many powders burn more consistently at higher rather than lower charges). I do 135 PF; it's as accurate as I can use, my Glocks like it, and I never worry about making declared PF.

Reliable Yes. Accurate Yes. I can ding 6 inch plates at 25+ yards. I do need to bench rest shoot some paper to say for 100% certainty. I do like the way N320 meters. I am working on my third load for 9mm. I just REALLY like the soft feeling of 7625 compared to Titegroup and N320. I like the N320 too, but not as much as the 7625.

Softer than 320? I'll maybe have to get some and give it a try.

[ben b.]

Having had too much statistics training, I also want small SDs. I am content with my typical SDs of 9 to 12 for 20-30 rd test samples.

If shorter rounds fxn properly in your pistola, you could shorten OAL from 1.155 down to 1.140, testing samples in .005 decrements. That frequently but not always tightens the SDs, especially in pressure-loving powders.

It's true that larger SDs don't correspond well (sometimes at all)with POI changes/variability, which is an endlessly vexing anomaly (for me). Too much other slop in pistols is likely the reason, but even when I shot rimfire benchrest, SD wasn't of use in ammo selection.

[Red Ryder]

Do they shoot well, by which I mean, do they feed reliably and are accurate? If so, maybe it's not worth worrying about, except the already very small chance of stringing together a few low rounds and not making the PF threshold (somebody with better math skills could probably give you the odds).

Lowering the SD takes a very high degree of consistency in everything that goes into the load. That includes all the individual components as well as the loading technique. Soooo, if you wanted to, you could use single head stamp same lot new brass, or same lot once fired all shot through the same gun, weigh and segregate single lot bullets by weight, use single lot primers, and load at a slow but steady and sustainable rate on a single stage press to eliminate the OAL variation that comes from using a multiposition shell plate. You could also change to a powder known to meter very consistently, like the tubular Viht types.

Having done just about all of the above, I'm not sure it's worth it. With a 6"x11" A zone, the need for super accurate ammo isn't there. That leave's PF, and raising the powder charge to a bit higher takes care of that, even if the SD doesn't shrink (though it might, since my impression is that many powders burn more consistently at higher rather than lower charges). I do 135 PF; it's as accurate as I can use, my Glocks like it, and I never worry about making declared PF.

Reliable Yes. Accurate Yes. I can ding 6 inch plates at 25+ yards. I do need to bench rest shoot some paper to say for 100% certainty. I do like the way N320 meters. I am working on my third load for 9mm. I just REALLY like the soft feeling of 7625 compared to Titegroup and N320. I like the N320 too, but not as much as the 7625.

Softer than 320? I'll maybe have to get some and give it a try.

Noticeably softer!

[G-ManBart]

Not buying it. If it were, lots of folks and companies would already be using it. Heck, AA&A loads Super with 7625, and if it were softer they'd be using it in their 9...but they're not.

[Red Ryder]

Not buying it. If it were, lots of folks and companies would already be using it. Heck, AA&A loads Super with 7625, and if it were softer they'd be using it in their 9...but they're not.

Ahhhhh! Beauty is in the eye of the beholder! At least that's what my momma always said. She also frequently stated that "you will never know until you try it". Liver still sucks. But anyways!

[CiDirkona]

Most of my SDs for my go-to standard loads are happily in the 12-15 range, so I personally wouldn't be TOO concerned with 19. I've had some poor powder choices (Longshot at low pressure) get the SD up to 40, and have had PowerPistol loading a 147grain lead bullet get the SD down to 7.0.

What can you try changing? Sort brass by hand? One thing I tend to do is seat a little deeper (if you're at a low charge on the min/max spectrum) to ensure that all the powder ignites (get every last cent out... ) My minor loads tend to be in the 1.090 length with lead 125s -- approximately the same internal volume as FMJs in the 1.115-1.125 range.

I'm a relatively new reloader though, so I'm looking forward to what everyone else has to say.

Edited April 14, 2011 by CiDirkona

[MoNsTeR]

1. sort brass by headstamp

2. use a powder that says "Vihtavuori" on the can

[Tom E]

For low SD's you need a powder that makes enough pressure, at the velocities you want, to burn efficiently/consistently. Too slow a powder and you'll see the SD's drop as you increase the powder charge past getting the velocities you want.

[7mmb]

I watch SD when I work up loads. I've found that many starting loads will yield high SDs and it will lower as powder charges increase and I find the powder/bullet combo's sweet spot. SD will again increase as you go too high and I know to back off on the charge. I've seen incremental increases in powder charges cause significant increases in average velocity and then as you approach the sweet spot AV won't go up but a few fps for the came increase in powder but the SD will drop dramatically. That's when you know you've found your load.

I don't look at SD in fps. Although my chrono displays it in fps it also displays it as a percentage of AV. I find it a better way to compare different loads at different velocities. An SD of 50 in a 1000 fps 9mm load would not be a good load but a 50 SD in a 3000 fps 223 would be fine. Factory ammo usually has an SD in the 2% to 2.5% range. If my handloads are down there I'm happy. I've had loads on the 1% range, but only with VV powders...

[mda]

So sorry for a noob question, but what does Standard Deviation do? Or represent in reloading?

±1 standard deviation is where 68% of the sampled data will fall around the mean. In the above example it means that 68% of the sampled bullets velocity will be ±20 fps from the mean. It also mean the remaining samples (32%) will be greater then ± 20 fps.

To the original question. In my experience reload 40, 45 or 38super and only making one change to lower the SD is to use Vit. powder. Like others have said you could experiment with crimp, bullet weight/type, seating depth, ect to find the sweet spot for a particular powder but that takes time and messing with settings. The only down sides to Vit. are availability (which where I live has not really been an issue) and cost. It is the cleanest burning most consistent powder I have ever used.

MDA

Edited April 18, 2011 by mda

[rhyrlik]

Using new brass, or brass that has been cleaned in stainless media to remove all residue from the innards, will result in a better case wall adherance to the bullet shank. This will make combustion more consistent.

[7mmb]

Using new brass, or brass that has been cleaned in stainless media to remove all residue from the innards, will result in a better case wall adherance to the bullet shank. This will make combustion more consistent.

I looked back through some of my old records when I was reloading 45 Colt today. I got lower SDs when my brass was new than I'm getting now after several reloads.

[Boxerglocker]

1. Same brass. 2. Good bullet. 3. Load needs to be efficient burns complete and clean, PF or 133 or higher IMO. 4. press need to be set-up for consistent powder drops and OAL.

My new Bear Creek Moly 135g WST loads 4.0g 1.150 oal averaged 133PF with a SD of 3.2 on the chrono this weekend.

[KyroWebs]

I wouldn't sweat SD too much, just as long as you know how to apply it to your load. I made a post on our local shooting forum a while ago and I thought I would copy/paste it here to explain how I use it when working up a new load.

Standard Deviation is like the bastard child of the chronograph. Not a lot of people really seem to know how to apply it to load development and I'll share how I do it. SD is basically a number that tells you how consistent your loads are. The lower the SD, the more consistent your loads are. Also, as the number of shots per string increases, SD will become a better indicator of consistency. (A SD in single digits using only 5 rounds as your sample isn't nearly as impressive as having a single digit SD using 15 or 20 rounds as your sample!)

Here is one string of a batch I worked up this afternoon out of my 610

939

959

960

957

958

951

964

967

955

955

961

955

High 967

Low 939

Average 956

Extreme Spread 28fps

Standard Deviation 7fps

Power Factor is (180*956)/1000=172.1 (180gr bullet)

Now comes the lesson on SD. Without going into too much detail, there is a ~68% chance that the velocity of your next shot will be within 1 SD of the average, ~95% chance that it will be within 2 SD of the average, and 99.7% chance that it will be within 3 SD of the average. As my own personal rule, I like to aim for a PF of 168 or 169 when I use (average velocity - 3*SD) In this example it would be 956-(3*7), or 935. Using 935fps as my average, I end up with a PF of 935*180/1000 = 168.3 PF. I'm happy with that.

This means that there is a 99.7% chance that I will be above 168.3PF with this load the next time I chrono my load. (and actually it is slightly higher than this since normal distribution also takes into consideration the probability of being 3SD over the mean) Keep in mind that this is using the same chronograph, with the same lighting, same outside temperature, altitude.....you get the point.

Edited April 21, 2011 by KyroWebs

[bbbean]

We talk a lot about means and SDs, and Z scores, but when you consider how our loads are measured at the match, these aren't necessarily the numbers we should be looking for. They give us a lot of information, and we CAN work from them, but we should consider how velocity is measured at the match: 3 rounds over the chrono, if the PF is low, three more and then the average of the three fastest of the 6.

Obviously, we don't want to simply test 3 or 6 rounds. The sample is far too low to authoritatively say we know those rounds were representative of the batch. So we increase sample size. That increases the odds that the sampled rounds are in fact representative of the batch and reduces the degree to which a single outlier can cause error.

So we test, say 20 rounds. We look at the mean to get a rough idea of whether our load is where we want it, and we look at the SD to see how consistent we are. Obviously, at any level, a smaller SD is better. But what we really need to do is to look at our slowest 6 rounds. Between the fact that in a random selection the slowest round has just as much chance of being samples as the fastest round, and Murphy's Law, what we really need to insure is that the mean of the fastest 3 of the slowest 6 rounds make PF. If we want to fully account for Murphy's law, we'l make sure the mean of the 3 slowest rounds make PF.

But that means we're still making a decision based on 3 rounds, so we need to repeat this a few times. While that may burn more ammo than you intended to, can you ever really have too much time practicing your trigger pull?

What I recommend: Run a minimum of 3 strings of 20 rounds over the chrono, with the goal of a mean 2 SDs above power factor. Be sure the average of the slowest 3 rounds in each string makes power factor, and then spot check when your have a time if you make a big batch. But if you're in a pinch, check the slowest rounds. If they're below PF, it won't matter that your man and SD say you SHOULD be higher!

BB