Powder Charge vs Standard Deviation

Tim

Checked Out
Staff member
2A Bourbon Hound 2024
2A Bourbon Hound OG
Charter Life Member
Benefactor
Vendor
Multi-Factor Enabled
Joined
Dec 17, 2016
Messages
16,370
Location
A Glass Cage of Emotion
Rating - 100%
85   0   0
I was messing around with different powder weights yesterday trying to squeeze a bit more velocity out of my 6.5CM.

I did all of the loading in a single session- same die setup, same brass prep, same powder lot, etc. so I'd expect that charge weight was the only variable in the process. I put together several loads a few 10ths of weight apart. I also waited ~20 minutes between strings to allow the barrel and suppressor to cool.

At my accuracy node (loads of 41.2 and 41.5), my SD was ~6fps.

As my charge weight went up, and I got out of the accuracy node, the SDs also started going up. Some in mid teens.

I've always assumed that SD was a good measure of your loading technique and consistency, and not necessarily dependent on the velocity, charge weight, etc.

Am I wrong? Is it normal to see SDs going south when you're outside the optimal charge weight?
 
Last edited:
I've been playing around with something similar. I loaded up various powder weights and just shot them "downrange" through a chrono and when the SD for a particular load was below 10 I shot for group. Lower SD's correlated with smaller groups. It makes sense since a low SD is a reflection of consistency in velocity which coincides with good loading technique.

So, you can actually shoot a string of various loads through a chrono and not have to worry about targets until you get a low SD. Then shoot for group.

Fun stuff!
 
It makes sense since a low SD is a reflection of consistency in velocity which coincides with good loading technique.


This is my question...is your statement actually true or just assumed to be so since it makes so much sense? My - admittedly small sample size - experience yesterday would say it's not true. All of my loads should have been equal, other than charge weight, but had significant differences in SD.
 
Last edited:
From what I've always understood, a low SD is a good indicator of precision (repeatability) but not always accuracy.

I've developed .223 600yd loads with SD in the high teens that could would hold the 10-ring, but it used up A LOT of the 10-ring (accurate, not precise example). Sufficient for SR shooting, but not anything like PRS.


9uImwvc.jpg
 
Last edited:
A change in charge weight will change the pressure and thus the velocity. This will change both the internal and external ballistics, even barrel harmonics, which translate into changes in velocity and group spread and ultimately SD.

Nothing is an exact science, and low SD's, or even high SD's, do not always translate into the best groups. But like said, it is a lot of fun to play around with this "stuff". I always learn something new.
 
So, are you saying that you can have both accuracy and precision with high SD's?
 
What kind of chrono are you using and what are the precision limitations of that device?
 
What kind of chrono are you using and what are the precision limitations of that device?

Magnetospeed v3. I have an MLOK mount for the chrono and it remained in place through all of the loads.
 
Last edited:
So, are you saying that you can have both accuracy and precision with high SD's?


Yes, absolutely. Here are the numbers and please note that all of the SD are double digit, some greatly so. Some of the ES are phenomenal and should have resulted in a lot of vertical stringing, but it remains absent on paper targets for all the years the loads are in use. (e.g none are sub minute of arc but all are reliable, predictable and hit where pointed with regularity in multiple platforms. I can't ask for much better, since for me consistent performance/predictability on target is worth more than overlaping holes)

Note, these loads are all maximum 2-3 min arc out to 200 yards. May not seem like much but they have proven them selves to be accurate at that level and usually much better in multiple rifles over the years. As you look at these, ES and SD can be all over the place and yet, paper up to 200 yards showed them all to hold their predictable levels of accuracy across time and rifles.

Numbers on my chrono are great, what counts is how I vet those numbers on paper at range. I no longer pay much attention to numbers inside 250 yards.

First load, from March 1999 and still in use today. Used in Win 70 Feather Weight, Mauser 93 Custom/ER Shaw barrel.

- 7x57 Mauser, 150g Barnes X flat base, Rem/Win/Fed case, Win LR Primer, 47.5g IMR 4831, 2.531" base to ogive. Average Velocity, 2637 fps, ES 84.1, AD 26.4, SD 32.5

Second load, from October 1994 and still in use today. Used in Mauser 98 Custom/Numric Barrel, Remington 700, Merkel K3. (Best 5 Shot test groups at 100 yds in the mauser and Remington ran 1.27" and 1.43")

- 30-06, 165g Hornady 3045/SPBT, Rem/Win/Fed case, Win LR Primer 51g H380, 2.741" base to ogive. Average velocity 2666 fps, ES 35.6, AD 11.4, SD, 14.4

Third load, from November 1998 and still in use today. Used in Type 44 Mosin Carbine, Viet Nam Bring Back, Iron Sights.

- 7x62x54, 180 Sierra Game King Spitzer, NNy/Hanson case, Win LR Primer, 51g IMR 4350. Average Velocity 2320 fps, ES 27.6 fps, AD 9.8 fps, SD 13.9

Fourth pair of load, from January 1997 and still in use today. Used in Remington Roller, IAB Roller, IAB Sharps, Shillen Barreled Sharps, 1886 Winchester, Uberti High Wall Big Game Rifle.

- 45-70, 405g lee FNHB, Win/Rem/Fed case, Federal LR Mag primer, 27.2g XMP5744. Average Velocity 1378 fps, ES 109.3, AD 15.7, SD 26.3
-45-70, 480g Lyman Schmitzer, Win/Rem/Fed case, Federal LR Mag primer, 25.5g XMP5744. Average velocity 1233 fps, ES 16.3, AD 5.0, SD 6.7

Fifth load, from May 1999 and still in use today. Used in Taylors Sharps. (typically 2 inches at 100 yards)

- 45-70, 350g Hornady JRN, Win/Rem/Fed case, Federal LR Mag primer, 47g Hodgdon H322. Average velocity 1870 fps, ES 172.4, AD 37.1, SD 49.7

Sixth load from June 2015 and still in used today. Used in 1886 Winchester and Uberti High Wall Big Game Rifle.

- 45-70, 405g Lee FNHB, Win/Rem/Fed case, CCI 200 LR Primer, 50.3g Hodgdon H380. Average Velocity, 1634 fps, ES 50, AD 18.3. SD 21.9.

Seventh Load from June 2003, sold rifle/no longer in use. Fired in CZ 550 bolt action. Typically under 2" at 150 yards.

- 9.3x62, 285g Norma JFP, Lapua Case, Federal 215 LR Mag primer, 66g Hodgdon H4350. Average velocity, 2360 fps, ES 209.5, SD 79.6
 
Last edited:
Magnetospeed v3. I have an MLOK mount for the chrono and it remained in place through all of the loads.

The accuracy of our chronographs can be expected to be between 99.5% and 99.9%.

I don’t recall the exact math behind this but if you are running 3000 fps then 0.5% is 15 fps. It’s possible that a change in SD from 6 to 15 is statistically insignificant, or close to it. I’d need to consult someone who remembers all that stuff from college.
 
Brian Litz says yes to OP's original question. Not only are optimal charge weights conducive to low ES/SD, optimal powder selection as well.
 
  • Like
Reactions: Tim
Heres another bit of fuel for the fire

You can have accuracy and precision without even KNOWING your SD/ES.

What would I do if I took my best shooting 224 Valkyrie load, shot it over a chrono, and the figures sucked compared to a different charge that didn’t perform downrange as well?

Some of these details can drive you insane. I got wrapped around the axle at first over all these details and it sucked the fun out of it. Now, I work up a load that won’t destroy my rifle or my face, that puts rounds where I want them at whatever distance I’m so inclined

There’s no harm in knowing at all. But I would suggest not becoming so tied up in it that it ruins the fun on the range
 
Not long ago, I started neck turning brass. I ran a quick experiment to see what effect it would have, if any, on standard deviation and accuracy. I ran charge weights in 0.2gn incrments, at total of 4 charge weights. I went two weights below my previous accuracy node, one was my accuracy node, and one was above. I ran both turned and non-turned loads so I could get a comparison. Five rounds each charge. 223, shot from a 20" AR.

As expected, the non-turned rounds lined up with my previous results, I had my accuracy node at the same charge weight, and the standard deviation was single digit (6.5). The other non-turned loads were double digit SD, from 13.2 to 16.0. The accuracy difference across the range was not significant, the lowest was 1.0 moa, the highest was 1.4. I would have to shoot more than 5 rounds at each charge before I could say there was any significant differences.

The surprise was when I tested the neck-turned rounds. The standard deviation was single digit for all of them, from 6.5 to 9.5. Extreme spreads for all hovered around 18. Accuracy was about the same as the non-turned rounds.

One conclusion I took from this is that neck turning definitely helped lower standard deviation across the whole charge range (more consistent neck tension), but did not improve the standard deviation of the optimum charge. Another possible conclusion is that the optimum charge is more tolerant of other variables (such as neck tension).

IMHO, a low extreme spread is not important at all if you are shooting rifle to 100 yards. When you are shooting to 600 yards, then it shows up as vertical stringing. To me, using a chrono to help develop loads gives important information, especially if reaching out past 300 yards. Knowing the exact velocity helps when calculating bullet drop. Otherwise, it is more trial and error to figure out drop for distances.

I also use a chrono for developing handgun loads for rounds that I don't intend to ever shoot past 25 yards. In that case, I am less worried about how the extreme spread affects accuracy (it really doesn't at 25 yards) but it does tell me if I am using a good powder for the task at hand. I've seen extreme spreads as much as 300fps when testing loads in 357 magnum when I do the powder-forward test.
 
Last edited:
Back
Top Bottom