Suppressor tightens SD of handload

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Morne
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Suppressor tightens SD of handload

Post by Morne »

Put the first rounds through my form 1 suppressor today! Very happy.

Here's what is weird, though. I was doing chrono work at the same time and gathered data on my load both with and without the suppressor. The standard deviations (SD) for the same exact loading with the suppressor were much, much tighter than without:
SD without suppressor = 27.1
SD with suppressor = 6.0

For those interested, it was 9x19mm Luger being fired from a pistol caliber conversion AR-15 (Olympic Arms upper, Sten mags, 16" barrel). The load was 3.0-gr W-231 behind a 165-gr X-Treme CPRN bullet primed with WSP(m) at approximately 1.160" overall length. Average velocities at about 10' from the muzzle:
FPS without suppressor = 855
FPS with suppressor = 852

Meteorological data:
39 F, 37% RH, 30.20" Hg

Thoughts on why the SD shrinks so markedly with the suppressor on the gun?
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Re: Suppressor tightens SD of handload

Post by Brian D. »

I'll try to remember to ask my shooting group next Sunday. A few have suppressors and chronographs.
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Re: Suppressor tightens SD of handload

Post by jeep45238 »

I''m guessing there's a more gentle transition from the barrel to the atmosphere, and it's a chamber that allows gas temperatures to stabilize and extra powder to burn.

I notice markedly tighter groups at 100 out of my 22 with a clean can vs. without.
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Re: Suppressor tightens SD of handload

Post by JustaShooter »

In addition to what Jeep said, I'm guessing your SD measurements are based on a small sample, perhaps 5 or 10 rounds. If so, normal variation in the loads may account for it. That is, by chance you were unlucky with the first batch and lucky with the second. (IMO, SD in a small sample like 5 or 10 rounds is almost meaningless, you really need a larger sample size. For those smaller batches, ES is a more useful statistic.)
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Re: Suppressor tightens SD of handload

Post by Morne »

JustaShooter wrote:In addition to what Jeep said, I'm guessing your SD measurements are based on a small sample, perhaps 5 or 10 rounds. If so, normal variation in the loads may account for it. That is, by chance you were unlucky with the first batch and lucky with the second. (IMO, SD in a small sample like 5 or 10 rounds is almost meaningless, you really need a larger sample size. For those smaller batches, ES is a more useful statistic.)
10 data points without can
7 with can

But see, a bunch of people gave me guff for reporting ES instead of SD when I was shooting 5 shot strings for my chrono work. Now here you’re going the other way. Grumble, grumble.

Doesn’t matter, the ES was smaller, too. Like 87 without and 17 with.
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Re: Suppressor tightens SD of handload

Post by JustaShooter »

Morne wrote:Doesn’t matter, the ES was smaller, too. Like 87 without and 17 with.
As expected with a small sample size.

Yes, I know I'm in the minority in the shooting community. Almost everybody is focused on SD - probably because somebody once spoke with authority on how important it was. And, they weren't wrong exactly, but almost always when people talk about their loads they are showing data from a string of 5, occasionally 10, shots. Without more data points SD just doesn't tell you much - whereas, ES can tell you a lot even in a small data set.

I'm not a statistician by any means, so I have to rely on what others tell me. But, my gut was screaming that ES in a 5-shot sample just couldn't be all that relevant, with my limited understanding of SD. So, I did some digging and came up with a forum post elsewhere from a statistics guy who is also a shooter. He said that for a sample size of 5, most of the time that will get you to an expected error rate within +/- 25fps (for pistol velocities). Quadrupling the sample size to 20 cuts the error in half to within 12.5fps. He also said that an estimate of SD based on 5 samples is not very precise at all, but ES *is* a good estimate of variation for small sample sizes so if you're using 5 shots, you might as well use ES instead of SD. If you want SD to be more relevant, you really need a sample size of 20 (but no need to use the 30 sample size guide often bandied about by statisticians as it won't reduce the expected error rate enough to be helpful.)

That said, you likely know (by watching ES) within 5 rounds or so if a load is worth pursuing further. These days, with component scarcity and cost being what they are, knowing when to stop after 5 rounds and when to go farther is pretty important.

I'll try to dig up that thread if I can...
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Re: Suppressor tightens SD of handload

Post by Morne »

Checked my data for ES:

Without can = 87.5 (10 data points)
With can = 18.2 (7 data points)
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Re: Suppressor tightens SD of handload

Post by JustaShooter »

Here's that thread about ES and SD: https://www.thehighroad.org/index.php?t ... nt.838540/" onclick="window.open(this.href);return false;
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Re: Suppressor tightens SD of handload

Post by weakhand luke »

When did all this start to matter?

(serious question)
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Morne
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Re: Suppressor tightens SD of handload

Post by Morne »

weakhand luke wrote:When did all this start to matter?

(serious question)
I just got into NFA toys, so for me the impact on a suppressor regarding the velocity of bullets (and the variation of said velocity) only recently began to matter.
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Re: Suppressor tightens SD of handload

Post by Brian D. »

weakhand luke wrote:When did all this start to matter?

(serious question)
It matters to precision shooters seeking maximum consistency, action shooters trying to make various power factors*, geeks who like crunching numbers, and anyone who fits in two or more of the above categories. :lol:

I would have been in category #2. IPSC, IDPA, and NRA Action all had minimum power factors that had to be met. Once in a while locally, and almost always at state level and above matches, your loads would be run across a chronograph. With bowling pin shooting, your ammo would have to be powerful enough to drive 3.5 lb. pins three feet back and off the table. I didn't always have access to pins and a table, but could take my chronograph along to check new batches.

*For those not familiar, power factor is: bullet weight (in grains) times velocity (feet per second) divided by 1,000. So, a 200 grain bullet going 900 feet per second, divided by 1,000, gives you a PF of 180.

I just used those numbers as an example by the way.
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Re: Suppressor tightens SD of handload

Post by weakhand luke »

I'm trying to imagine all the things that might affect ES, besides the can. Gil Hebard in his "Pistol Shooter's Treasury" makes note that the brand of primer has a significant affect. Also, lead alloy, powder lots, temps and humidity might play into this? Bullet crimp and seating depth? Would type of rifling do anything to the ES, all other things being equal? Does ES change if the firearm was rigidly mounted vs handheld, perhaps? Limp wristed? Does a tenth of a grain difference in powder charge matter? .01 grain? Full jacket, half jacket, plated, coated, or lead better? Does type of lube have a big affect?

If 0 fps ES were acheivable, who can predict how one's scores may improve (50' bulleye) over an ES of, say, 100 ? (K-38, 148gr HBWC, 2.5grs BE)

I love this stuff.
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