The Firing Line Forums

Go Back   The Firing Line Forums > The Skunkworks > Handloading, Reloading, and Bullet Casting

Reply
 
Thread Tools Search this Thread
Old November 23, 2021, 05:24 PM   #1
Shadow9mm
Senior Member
 
Join Date: March 21, 2012
Location: Indianapolis, IN
Posts: 2,059
How many rounds for relevant chronograph data?

So I have seen some interesting digressions in posts in the past as to the statistical relevance of chronograph testing in relation to how many rounds were fired and whether they were meaningful or not.

My questions is at what point does the data become useful, and at what point does it become statistically relevant?

Does this vary between rifles and handguns?

For handguns I generally shoot 5rnds each to test for velocity and consistency when working a load up. After that I generally shoot 10-20 at my chosen charge weight to confirm the data. While I understand that 5 or 10-12 are not statistically relevant, are they useful. Would I get more meaningful data shooting more, say 10rnds each during the workup, or 25-50rnds during confirmation? At what point do you decide you are just splitting hairs and wasting components in these times of shortages?
__________________
I don't believe in "range fodder" that is why I reload.
Shadow9mm is offline  
Old November 23, 2021, 05:31 PM   #2
nhyrum
Senior Member
 
Join Date: March 9, 2016
Posts: 541
I believe to get an accurate standard deviation, you should have at least 8 data points. 5 is my minimum, but I usually shoot 10. That gives me room to throw out the highest and lowest and still have 8.

For rifle, I try to Chrono whenever I can even after load development. It helps me track velocity vs temp so I can get an accurate fps/degree graph, but that's probably more info than most care about

Sent from my SM-G996U using Tapatalk

Last edited by nhyrum; November 23, 2021 at 05:38 PM.
nhyrum is offline  
Old November 23, 2021, 06:28 PM   #3
stagpanther
Senior Member
 
Join Date: March 2, 2014
Posts: 9,473
I have found that there is an inverse correlation between the acceptable group size and the amount of days before I run out of primers.
__________________
If you’re ever hiking in the woods and you get lost, just look up and find the brightest star in the sky and you’ll know which way space is.
I am NOT an expert--I do not have any formal experience or certification in firearms use or testing; use any information I post at your own risk!
stagpanther is offline  
Old November 23, 2021, 06:31 PM   #4
Shadow9mm
Senior Member
 
Join Date: March 21, 2012
Location: Indianapolis, IN
Posts: 2,059
Quote:
Originally Posted by nhyrum View Post
I believe to get an accurate standard deviation, you should have at least 8 data points. 5 is my minimum, but I usually shoot 10. That gives me room to throw out the highest and lowest and still have 8.

For rifle, I try to Chrono whenever I can even after load development. It helps me track velocity vs temp so I can get an accurate fps/degree graph, but that's probably more info than most care about

Sent from my SM-G996U using Tapatalk
agreed, for rifle I always try to chrono before I shoot each time if I can just to re-verify. Im thinking more towards working up a load. If your initial data is bad, it could throw the whole workup in theory.
__________________
I don't believe in "range fodder" that is why I reload.
Shadow9mm is offline  
Old November 23, 2021, 07:05 PM   #5
totaldla
Senior Member
 
Join Date: August 10, 2009
Posts: 1,064
For handgun, I usually measure 5 or 6 just to make sure the velocity is about where I thought it should be. I might shoot a long string ( 10+) if I think the variation is not as tight as it should be.
totaldla is offline  
Old November 23, 2021, 07:58 PM   #6
hounddawg
Senior Member
 
Join Date: March 1, 2009
Posts: 3,991
For initial load workup I use three rounds. I just look for flat spots in the curve. Works for me
__________________
“How do I get to the next level?” Well, you get to the next level by being the first one on the range and the last one to leave.” – Jerry Miculek
hounddawg is offline  
Old November 23, 2021, 08:17 PM   #7
rclark
Senior Member
 
Join Date: February 12, 2009
Location: Butte, MT
Posts: 2,445
For my handgun testing, I always shoot 15 per load. 10 is minimum sample group for SD from my limited understanding of statistics, although I pay more attention to ES. For awhile I was shooting 30 rounds, but found that unnecessary even though 'more' is obviously better. Most cases you'll find SD is just about 1/3 of ES anyway. I like shooting 15 because it allows me to throw out obvious flyers if necessary. Ie. is all are within 20fps, but one is out at 50fps ... toss it out. Also if chrono has an error, again, no biggie, still have a good sample group. Also easier to tell on paper where all the shots are going on average. Might say is just gives me a better feel for the load... Just me though. Most times I'd have a box of 100 ready for testing when I go out to test a new powder. Give me 6 tests with 15 and a final test with 10. Works for me.
__________________
A clinger and deplorable, MAGA, and life NRA member. When guns are outlawed, only outlaws will have guns. Single Action .45 Colt (Sometimes colloquially referred to by its alias as the .45 'Long' Colt or .45LC). Don't leave home without it. That said, the .44Spec is right up their too... but the .45 Colt is still the king.
rclark is offline  
Old November 23, 2021, 11:43 PM   #8
Metal god
Senior Member
 
Join Date: April 10, 2012
Location: San Diego CA
Posts: 6,029
During load development I shoot 5rd groups at each charge weight . I generally will not chono during this stage unless I've not used the components together before . Once I've found a charge or two I like that's when I start chrono'ing my loads to be sure I'm not way off a node but still won't chrono everything . Once I've settled on a load I'll take out 30-ish rounds and shoot two 10 shot groups for accuracy and one 10 shot group through the chrono to confirm everything . I do it that way because I've never like shooting through/over a chrono while also trying to shoot for accuracy . So when I shoot through the chrono I'm trying more to get perfect alignment and rifle hold then I am a great group .

Hmm that was a lot to say when I could have just said 10 shots is good lol .
__________________
If Jesus had a gun , he'd probably still be alive !

I almost always write my posts regardless of content in a jovial manor and intent . If that's not how you took it , please try again .
Metal god is offline  
Old November 24, 2021, 04:00 AM   #9
kilotanker22
Senior Member
 
Join Date: October 14, 2012
Location: North Central, PA
Posts: 1,817
I use five shot groups for initial load development. I chronograph while I shoot for groups for extra data. At this point though, groups are not my main focus. I find a load with good es and SD (es 20 fps or less with a single digit SD ). Then I adjust seating depth to tune the load to the gun, chronographing the whole time. That way I can see if something is being affected by the change in seating depth.

Been using this method for about a year now, seems to work well for me.
__________________
“We do not seek peace in order to be at war, but we go to war that we may have peace. Be peaceful, therefore, in warring, so that you may vanquish those whom you war against, and bring them to the prosperity of peace.
– St. Augustine
kilotanker22 is offline  
Old November 24, 2021, 06:04 AM   #10
mehavey
Senior Member
 
Join Date: June 17, 2010
Location: Virginia
Posts: 6,149
Initial Rough: 3 rounds
If ES ≤ 2% of Average, call it a day for general purposes
If greater than 2%, five rounds.

The toughest standard in life is "good enough"
mehavey is offline  
Old November 24, 2021, 10:06 AM   #11
hounddawg
Senior Member
 
Join Date: March 1, 2009
Posts: 3,991
I have gotten over ES/SD numbers, now the only reason I chrono is to look for flat spots when doing the charge test and a couple of shots after deciding on a seating depth for scope come ups. I am using a electronic target now and can see each individual bullet impact and it's velocity and have seen my lowest FPS at the chrono hit highest on the target and vice versa. Barrel Harmonics? Shooter error?
__________________
“How do I get to the next level?” Well, you get to the next level by being the first one on the range and the last one to leave.” – Jerry Miculek
hounddawg is offline  
Old November 24, 2021, 01:27 PM   #12
FlyFish
Senior Member
 
Join Date: January 20, 2009
Location: Overlooking the Baker River Valley
Posts: 1,713
There is no minimum sample size for calculating standard deviation (well, you do need at least two measurements), nor is there any magic number of samples at which standard deviation becomes statistically valid, or useful, or significant (a term in statistics that means something very different from the way most people use it). What a larger sample size, and the sd calculated from that larger sample size, does is allow more specific statements to be made about the population that the samples were drawn from.

If, for example, you're interested in determining the velocity of a particular load, what you really want to know is not the average velocity of your samples, but rather the [unknown, and unknowable] average velocity of the theoretically infinite population of possible samples. The sd can be used with your sample average to place bounds (confidence limits) on that true population average, and the more samples the sd is based on, the narrower those bounds will be.

It's never valid, by the way, to toss out any samples (high and low, for example), unless you can be absolutely sure they are not part of the same population. For example, if you had a squib that triggered the chronograph but otherwise somehow must have had a reduced powder charge, and you were sure about that, you would probably be justified in tossing that data point. (Some statisticians I know would argue even that would not be OK).
__________________
NRA Benefactor Life Member
NRA Certified Instructor: Rifle, Pistol, Shotgun, Metallic & Shotgun Shell Reloading; RSO
Pemigewasset Valley Fish & Game Club
FlyFish is offline  
Old November 24, 2021, 01:35 PM   #13
Shadow9mm
Senior Member
 
Join Date: March 21, 2012
Location: Indianapolis, IN
Posts: 2,059
I never toss any rounds from my chrony or groups, what you shot is what you shot
__________________
I don't believe in "range fodder" that is why I reload.
Shadow9mm is offline  
Old November 24, 2021, 05:20 PM   #14
GeauxTide
Senior Member
 
Join Date: April 20, 2009
Location: Helena, AL
Posts: 4,105
In almost 50 years of using a Oehler 33 and 35p, I have used 10 rounds as the benchmark to get accurate Standard Deviation and Extreme Spread. Any gun writer that I've read during this time has done the same.
__________________
Reloading For: 223R, 243W, 6.5GR, 6.5 CM, 260R, 6.5-06, 280R, 7mmRM, 300HAM'R, 308W, 30-06, 338-06, 9mm, 357M, 41M, 44SPL, 44M, 45 Colt, 450BM.
GeauxTide is offline  
Old November 25, 2021, 06:44 AM   #15
std7mag
Senior Member
 
Join Date: June 23, 2013
Location: Central Taxylvania..
Posts: 3,356
I don't use the chrono during load development, usually.
I do a ladder test then adjust seating depth.
I don't want to get too hung up on velocities when looking for accuracy.

The exception is when doing some of my "obsolete" cartridges with powder/bullet combos that i cannot find load data for.
Example would be my 284 Win using Alliant PP4000MR.

After i've done my load development, i'll run 5 shots over the chrono. If it doesn't look acceptable to me, i'll find a different load. If i like where the numbers are pointing, i'll shoot 5 more over the chrono.
Then i'll shoot 600-800 yards to ture my velocities.
__________________
When our own government declares itself as "tyrannical", where does that leave us??!!
std7mag is offline  
Old November 25, 2021, 09:08 AM   #16
Shadow9mm
Senior Member
 
Join Date: March 21, 2012
Location: Indianapolis, IN
Posts: 2,059
I try to find loads that are consistent in velocity, and tune seating depth in that area.
__________________
I don't believe in "range fodder" that is why I reload.
Shadow9mm is offline  
Old November 26, 2021, 06:39 PM   #17
RC20
Senior Member
 
Join Date: April 10, 2008
Location: Alaska
Posts: 6,734
This came up a bit ago. The one answer was 50 rounds for SD.

Frankly I think its mostly if you love geek stuff and (in this case, pun) statistics.

Me? What I want is a ball park velocity (rifle only, I don't care about pistol). The 06 for my current load will have an ES of 100 or so, have to look up the SD.

Still shoots 3/4 MOA. the 6.5 Lapua seems to shoot best at under 50 ES and more like 20 SD. Shrug.

One idea (for me) is to duplicate speeds with other powders. Seems to be fairly close to accuracy nodes though there also seems to be a bit of shift.

I am also logging temperatures to see what that does or does not, in a casual way.

I target shoot for fun so its more interesting and some looks at correlations (the 6.5 if it throws one and it feels like a good shot is off the normal average FPS wise)
__________________
Science and Facts are True whether you believe it or not
RC20 is offline  
Old November 28, 2021, 04:04 PM   #18
Unclenick
Staff
 
Join Date: March 4, 2005
Location: Ohio
Posts: 19,283
RC20's observation can depend on the gun rather than the cartridge, based on the rate of muzzle rise that occurs while the bullet is still in the barrel.

The answer to the OP is that it depends on how certain you want to be that your answer is close to correct.

Correct what, exactly?

That takes a little explaining. The calculation your chronograph calls SD or just "s" is an estimate. What it estimates is the standard deviation of an infinite size sample (aka, a huge population) of the same thing. It's the standard deviation you are estimated to have if you could fire an infinite number of rounds of the same load in the same gun under the same conditions without that gun experiencing any wear. The error in the estimate narrows as the sample size increases because each additional sample is more likely to be representative than it is to be an outlier. So a larger sample accumulates a greater preponderance of representative members.

The statistical reason for making that estimate of what an infinite sample will do is to predict the future behavior of that ammo. Things like how many outliers you can expect and how often their different values will occur, or what an average group of x shots on a target will look like or to decide if one outlier in your velocities is likely to be so rare it should not be counted, etc. This 3 min video explains it well. (and velocity does tend to have the normal distribution mentioned in the video). The practical reason for measuring velocity SD is that it is an indicator of consistency that can help with long-range accuracy, in particular, and help troubleshoot a number of problems that affect accuracy, especially irregular or inadequate primer seating, or powder that's going bad.

So, what sample size to use? SAAMI standards use 10 rounds for pressure standard deviations, but they are premised on a maximum allowed standard deviation (4% for HP rifle cartridges and 5% for handguns) plus two standard errors in the result. The two standard errors add 2.53% to the rifle average pressure and 3.16% to the handgun cartridge average pressures. The addition is to cover the fact the next set of ten is allowed to have an average value up to that greater total on the plus side.

The standard error is found by dividing the standard deviation by the square root of the number of rounds in the sample. It is a useful number because it is the estimated standard deviation of the mean result. It tells you 68% of the future 10-shot groups will have a mean (average) value within ±1 standard error and that 95% will be within ± two standard errors. So what you can do is take the standard deviation and divide it by the square root of your number of shots and see if that much error is acceptable to you? The bigger the number of shots in your group, the bigger that square root gets so, if you assume the standard deviation was estimated accurately by each calculation for the different sample sizes it will be the same for them all, so the error will get smaller as the group size increases.

The extreme spread will also vary with sample size. Ideally, it would not change the SD calculation, but in reality, as the group size gets too small, the odds of it accurately and uniformly representing the population get smaller. This leads to a problem with accuracy, too. The table below shows how extreme spread can be expected to vary with sample size out to the 95% limits. In other words, shooting multiple samples, expect 95% will stay somewhere between the red and blue lines where they cross the vertical line for your sample size.



Also, it is normal for your extreme spread to get bigger, on average, with larger samples because a larger sample offers more opportunities for the less probable outlying values to occur. That change will look like this:



So, when you increase your sample size from 3 to 10, the first chart shows the sample-to-sample ± scatter decreases, but the second chart shows the extreme spread will, on average, almost double. If, for example, the extreme spread in question was group size, your group sizes would be less different from one group to the next by a factor of 4, but the average size would be almost twice as big as your three-shot group sizes were averaging.
__________________
Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Rifle Instructor
NRA Benefactor Member and Golden Eagle
Unclenick is offline  
Old November 28, 2021, 04:43 PM   #19
rclark
Senior Member
 
Join Date: February 12, 2009
Location: Butte, MT
Posts: 2,445
Quote:
it depends on how certain you want to be that your answer is close to correct. Correct what. exactly?
Thanks for the refresher and graphs. Two semesters of stats in college back in the 80s was enough for me to know I didn't want to enter that field for a living . It was good though to know how/why you use certain sample sizes to increase your 'confidence' in the results.... Not just for chrono results but when politicians, groups, start spouting stats, poll results, etc. .....
__________________
A clinger and deplorable, MAGA, and life NRA member. When guns are outlawed, only outlaws will have guns. Single Action .45 Colt (Sometimes colloquially referred to by its alias as the .45 'Long' Colt or .45LC). Don't leave home without it. That said, the .44Spec is right up their too... but the .45 Colt is still the king.
rclark is offline  
Old November 29, 2021, 07:40 PM   #20
9MMand223only
Senior Member
 
Join Date: November 30, 2014
Posts: 213
I think how many shots depends on what you are looking to do. Benchrest is different answer than hunting is different answer to 3 gun.

If you load quality ammunition you get get good enough data from 3 shots I would say for almost every purpose except benchrest. If I competed in bench-rest, I would test 10 string minimum.
9MMand223only is offline  
Old November 29, 2021, 08:46 PM   #21
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 8,681
Most interesting to me is arsenal's accuracy tests for 30 caliber match ammo was 270 shots from barreled actions laying in a Mann rest. Bullets from 3 or 4 different die sets were used. Mean radius had to be under 3.5 inches at 600 yards.

It's also interesting that the most accurate benchrest stuff at 100 yards shoots almost .3 inch extreme spread.

Last edited by Bart B.; December 2, 2021 at 01:01 PM.
Bart B. is offline  
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 05:41 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
This site and contents, including all posts, Copyright © 1998-2021 S.W.A.T. Magazine
Copyright Complaints: Please direct DMCA Takedown Notices to the registered agent: thefiringline.com
Page generated in 0.05657 seconds with 9 queries