Reply With QuoteHaving read many of your posts I would think your experience at this is in the top 5% on this forum. A standard deviation of .01 +- sounds very consistent to me. How does this compare with all your experimenting with the 500.
What's a good standard deviation?
When loading pistol ammo, what std. deviation should I be "shooting" for?
What's good, mediocre, and bad?
Obviously, lower is better, but what's a good number?
For example, today I chronoed some new 9mm loads (6.0grs Power Pistol w/124gr FMJs). Ten shot groups had an average velocity of 1225fps, an extreme spread of 42 fps and a std deviation of 13.
Is that good?
Having read many of your posts I would think your experience at this is in the top 5% on this forum. A standard deviation of .01 +- sounds very consistent to me. How does this compare with all your experimenting with the 500.
Yeah, that.^^^Originally Posted by Manomet
Your numbers are better than factory Speer Gold Dot +P 124gr, if that's any indication.
http://www.rothenbuhler.com/gun/golddot.htm
I'm sure that you know, but for the benefit of others, low SD does not mean accuracy by itself.
I look at SD only as an indication of whether a load has room for improvement.
I would not change a load with +/- 1% velocity variations if it consistently shoots well.
On the other hand, a load with a decent velocity SD that still doesn't shoot well would tell me to look for problems with the bullet, not the powder charge.
These are the things that make handloading an interesting hobby.
Jack
Thanks, but I think you're mistaking arrogance for ability.
Let's hear from the top 1%:
Jack,
That's what's so friggin baffling. The loads above were inaccurate. I was lucky to get 4" groups at 10 yards with a G17 (which I normally shoot well). I switched to a S&W 5606 (which I seem to always shoot well) and saw similar results.
Just to make sure I could still shoot, I switched to an ancient S&W M&P (.38 SPL revolver) with 148gr plated DEWCs, avg velocity of 644, SD of 40(!) and put 12 shots into one big ragged hole.
I figured that maybe it was a "revolver day" so I tried the 9mms in a S&W 547 (which is essentially a 9mm Model 10) and got a 3-1/2" group.
The DEWCs above are the second most accurate loads I make. The most accurate are some for the .460 with an average velocity of around 1950 and a standard deviation of under 5.
EDIT: The 9mms were not crappy bullets. They were FMJ Winchesters that measured exactly .355" and weighed within a couple of grains of each other (at least the 20 or so that I weighed after the fact did).
Last edited by EddieCoyle; 04-24-2008 at 09:29 PM.
You raise a complex question, and one for which, so far as I can divine, there is at the moment no clear answer.
Let us start with the basics. Standard deviation is a statistical measure of the implied tendency of members of a “population” to deviate from the mean, based on the observed tendency of members of a “sample” to deviate from the mean.
With respect to ammunition, if we hypothesize that exactly identical reloading will yield exactly identical velocity, Standard Deviation implies a measure of the degree to which reloading was not exactly equal (i.e., was a bit sloppy).
However, the hypothesis is too simple, because even if you could make two exactly identical rounds, factors other than a difference in their construction could produce a variation in their measured velocity: for example, grain-to-grain variability in the combustion velocity of propellent, round-to-round variability in the brisance of primers, small but significant differences in headspacing, differences in ambient and in chamber temperature, the effect of the firing of a prior round on the mechanical impedence incurred by the bullet of a subsequent round traveling through the same bore, and probably a whole bunch of others.
At the same time, we hypothesize that consistency or variability of bullet velocity has a direct effect on group size (at least in terms of vertical spread). This makes sense, at least theoretically, because all other things being equal bullet velocity at the point of exiting the muzzle directly affects the shape of the ballistic curve.
And yet we have seen too many consistent reports where loads with a higher SD (implying a greater variability of muzzle shot-to-shot muzzle velocity) print smaller groups than loads with a lower SD. The vice, it seems, lies in the qualifier “all other things being equal.”
Apparently, “all other things” are not equal, and the problem is that no one seems to have a firm idea (or, better yet, some empirical proof) of what they are.
Which leads to this conclusion: when the SD of measured shot-to-shot velocity achieves a certain lower limit, it is safe to assume that other factors affecting group size have become dominant, and the quest for greater consistency of velocity (i.e., a lower SD) is a waste of effort. What is that lower limit? Since no one else knows for sure, I certainly can’t claim to, but I’d venture it reasonable to assume the line is somewhere in the range of 1-2% of nominal muzzle velocity.
Just for fun, roll a handfull across the table and see if the bullets are seated straight. A chunk of crud stuck in the seater or a poor match between the bullet shape and the seater plug might cause a problem.
Jack
Funny. I don't shoot a ton of 9mm but that IS my load. Here is my data. I'm very surprised you are getting such velocity.
Sig P228, 124gr Berrys, 6.0gr PP, WSP primer, 1.130" COL, 80 degrees, 7/17/2007
Hi 1092
Lo 1059
Av 1076
Es 33
Sd 10
Thanks for the explanation RKG.
I think it's the bullet shape. The Winchester bullets that I'm using have a longer, pointier nose and have less bearing surface than the Berrys.
Since they're longer, they seat deeper in the case (and I loaded them a bit shorter than you at 1.115") than the Berrys and run at a higher pressure. Also, less bearing surface means less friction. At least that's my theory, and I'm sticking to it.
FWIW, the same rounds ran about 75fps slower through a Luger than they did through a G17.
I shot all the rounds I loaded, but I did load some new ones last night with a lighter charge (5.7gr as opposed to 6.0gr) and those rolled pretty good. It doesn't surprise me, I'm using a Hornady seating die with the sliding collar that pre-aligns the bullet before seating.
Last edited by EddieCoyle; 04-25-2008 at 05:17 AM.
This sounds just like what I read about in Lee Relaoding second edition pg 136. They are talking about cast bullets and compressive strength in regards to chamber pressure, I don't know if fmj's are so much stronger it doesn't pertain but it clearly shows if you get to much chamber pressure and go beyond the comp. strength of the bullet accuracy goes down. Speer # 13 talks about different barrels and guns have varying effects on both press. and velocity. Just a thought
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