Book for relationship between SD and volume etc

L. Boscoe · January 15, 2025, 10:00 PM

I am hoping the wise ones on this forum will know: Does there exist a documented relationship between case volume and standard deviation in bullet speed?
I am thinking my current project may be having higher SD's because I am trying to load my 38 supers to a lower
speed than it was designed for.
With 3.9gr of VV N320, I am getting 825 to 925 fps with
a 125 grain bullet, Zero JHP. Of the 24 shots so far I am
getting 40 to 60 SD.
Next step is to shoot a 158gr bullet and see if that helps.
It would be great if there is a proven relationship, just asking.

**44 AMP** · January 16, 2025, 03:46 AM

there is an observed general relationship, but I don't know of any written studies, possibly because it doesn't seem to be a consistent linear thing.

Before you get all twisted up in SD and ES, what are your down range results??

Things that make a noticeable difference in a bench rest gun don't always matter when shooting a pistol at shorter ranges.

Sure, the goal is always to have every round as identical to each other as possible, but there are many times when a measurable difference is not a significant difference at the target.

74A95 · January 16, 2025, 12:43 PM

As 44 AMP noted, what's important is how well they shoot, not what the SD and ES are.

FWIW, there is no obvious relationship between SD and ES on accuracy at 25 yards;

https://americanhandgunner.com/handg...city-accuracy/

Jim Watson · January 16, 2025, 01:21 PM

A standard deviation of 40-60 implies an extreme spread of 150 fps, which I consider too much.

The gunzine article is working at much more consistent levels than the OP here.
His coefficient of variation - SD as a percentage of the average - is 1.06%. A PhD of my acquaintance got interested in reloading, shot a good deal, and concluded that a CoV of 1% was "match grade."

Dillonhelp · January 16, 2025, 02:27 PM

If the N320 charge leaves the case mostly empty, it could be the primer isn't igniting the powder all at once. First, be sure you are using the same brand and lot of cases. Next, try a hotter primer and see if the SD changes. Try a Federal primer, or switch to a magnum pistol or even a small rifle primer.

**Unclenick** · January 16, 2025, 03:16 PM

Yep! Magnum primers can make a big difference when most of the case is empty. Deburring flash holes can also help with some powders (spherical, especially) but isn't consistently helpful with other powder types.

Since his ES is 100 for an average SD of 50, I am going to guess the OP is providing average results for 8 samples of 3 as read from his chronograph. ES is expected to be three times SD when the sample size is around 9, but at 3 samples the value is lower. The correction factor is the statistic ξ(n) divided by the statistic C4(n). C4(n) corrects for square root bias in the method of calculating SD used in every chronograph I've ever seen.

L. Boscoe · January 16, 2025, 09:42 PM

case is half full, maybe 60% full. I may have mag small primers,
that is a good idea. The article was very informative

January 15, 2025, 10:00 PM	#1
L. Boscoe Senior Member Join Date: October 30, 2021 Posts: 329	Book for relationship between SD and volume etc I am hoping the wise ones on this forum will know: Does there exist a documented relationship between case volume and standard deviation in bullet speed? I am thinking my current project may be having higher SD's because I am trying to load my 38 supers to a lower speed than it was designed for. With 3.9gr of VV N320, I am getting 825 to 925 fps with a 125 grain bullet, Zero JHP. Of the 24 shots so far I am getting 40 to 60 SD. Next step is to shoot a 158gr bullet and see if that helps. It would be great if there is a proven relationship, just asking.

January 16, 2025, 03:46 AM	#2
44 AMP Staff Join Date: March 11, 2006 Location: Upper US Posts: 30,495	there is an observed general relationship, but I don't know of any written studies, possibly because it doesn't seem to be a consistent linear thing. Before you get all twisted up in SD and ES, what are your down range results?? Things that make a noticeable difference in a bench rest gun don't always matter when shooting a pistol at shorter ranges. Sure, the goal is always to have every round as identical to each other as possible, but there are many times when a measurable difference is not a significant difference at the target. __________________ All else being equal (and it almost never is) bigger bullets tend to work better.

January 16, 2025, 03:16 PM	#6
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,743	Yep! Magnum primers can make a big difference when most of the case is empty. Deburring flash holes can also help with some powders (spherical, especially) but isn't consistently helpful with other powder types. Since his ES is 100 for an average SD of 50, I am going to guess the OP is providing average results for 8 samples of 3 as read from his chronograph. ES is expected to be three times SD when the sample size is around 9, but at 3 samples the value is lower. The correction factor is the statistic ξ(n) divided by the statistic C4(n). C4(n) corrects for square root bias in the method of calculating SD used in every chronograph I've ever seen. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

January 16, 2025, 12:43 PM	#3
74A95 Senior Member Join Date: November 26, 2016 Posts: 1,674	As 44 AMP noted, what's important is how well they shoot, not what the SD and ES are. FWIW, there is no obvious relationship between SD and ES on accuracy at 25 yards; https://americanhandgunner.com/handg...city-accuracy/

January 16, 2025, 01:21 PM	#4
Jim Watson Senior Member Join Date: October 25, 2001 Location: Alabama Posts: 19,187	A standard deviation of 40-60 implies an extreme spread of 150 fps, which I consider too much. The gunzine article is working at much more consistent levels than the OP here. His coefficient of variation - SD as a percentage of the average - is 1.06%. A PhD of my acquaintance got interested in reloading, shot a good deal, and concluded that a CoV of 1% was "match grade."

January 16, 2025, 02:27 PM	#5
Dillonhelp Member Join Date: February 9, 2007 Location: Arizona Posts: 70	If the N320 charge leaves the case mostly empty, it could be the primer isn't igniting the powder all at once. First, be sure you are using the same brand and lot of cases. Next, try a hotter primer and see if the SD changes. Try a Federal primer, or switch to a magnum pistol or even a small rifle primer.

January 16, 2025, 09:42 PM	#7
L. Boscoe Senior Member Join Date: October 30, 2021 Posts: 329	case is half full, maybe 60% full. I may have mag small primers, that is a good idea. The article was very informative