How accurate is your powder measure?


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nvshooter
December 22, 2006, 01:36 PM
I bought the blue Chrony chronograph about two months ago and have had it out to the dry lake a time or two. It gives the statistics for each of six ten-round strings-- things like high and low shot velocities, the spread between these two, average velocity, standard deviation, et cetera.

I began to wonder about statistics after I got this thing. I did a few websearches and learned that Standard Deviation is a measure of the "normalness" a sample is among the population. At least I think this is what it means. I failed my Stats class in college back in '84...

Then it occurred to me that anything that involves multiple samples could be measured for its stats. I found a website that has a standard deviation calculator and began to fool with it. You tell it how many samples you have, put in the values and hit Caculate. It will give you multiple stats on the values in just split seconds...

So last night I threw 20 powder samples on my Lee Perfect Powder Measure at a weight that is pretty much in the ballpark for my 300WSM cartridge. I arbitrarily picked about 60 grains or so of H380. I threw the charge, then weighed it on my RCBS Range Master 750 digital scale. I recorded the weights and entered them into the calculator.

For my 20 weights, the mean was 60.9 grains. The standard deviation was .250 (assumably grains). The median was 60.9 grains. The HI was 61.3 grains; the LO was 60.4 grains. Something called the "95% confidence interval for actual mean" was 60.77 thru 61.00. I think this means I have a 95% chance of throwing a charge between 60.77 and 61.0 grains, but I'm not sure. Something else called the "average absolute deviation from median" came in at .185 (assumably grains). I think this means I can expect to be .185 grains off from the median for any thrown weight. Again, I flunked Stats, so I don't know.

If anybody out there is a Stats major, I'd be gratified if you'd explain in verbiage a trucker can understand what I have in these values. I think I have a pretty repetitive measure in that the SD is just 2.5 over 609, so that's pretty good, huh? Thanks.

Get the calculator at http://www.physics.csbsju.edu/stats/cstats_NROW_form.html

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Ben Shepherd
December 22, 2006, 01:43 PM
Seems that that measure works just fine with that particular powder. You're good to go.

Eagle103
December 22, 2006, 01:46 PM
I took college stats a couple of years after you but my recollections of all the various terminologies isn't there. I did manage to pass though.:D
The numbers you give sound pretty good to me. These volumetric throwers do amazingly well IMO. But as you probably already know if you want to be real precise you need to throw a little light and trickle the rest.

nvshooter
December 22, 2006, 02:09 PM
I do trickle my charges up to exact weight-- now. I was just using the measure before I got my target rifle and subsequently got insane about consistensy and predictability. I was using a Lee beam scale back then to set the measure and noticed that some charges were just slightly heavier or lighter than others when I'd do a ten-round or twenty-round recheck. I got a Redding trickler and the digital scale. Then I saw the light...

cracked butt
December 22, 2006, 07:01 PM
With the way your powder measure is throwing charges, its a waste of time to weigh out individual charges-try it both ways for comparison some time.

You'd be better served to spend that time weighing your brass and ensuring consistancy among cases.

30Cal
December 22, 2006, 07:45 PM
Those are pretty good numbers for as-dropped charges. Standard deviation I see is about +/-0.2 grs on mine with an extreme spread out to 0.4grs. I don't typically weigh individual charges, but I've done enough work with the scale to develop a technique to get good repeatable drops from the measure.

I do weigh each of the charges I shoot at 600yds in competition, but mostly because everyone else does. I'm still not sure it makes a difference.

Clark
December 23, 2006, 02:25 PM
My RCBS uniflow is perfect with many powders but...

1)crunchy with IMR4895
2) so inaccurate, it ain't worth a toot with 800X

shooting on a shoestring
December 23, 2006, 02:37 PM
Oh I wish I'd had my chronograph years ago in the 70's. Its taught me alot. I do find that my bench rested rifle groups are tightened by trickling to weight. But for hunting loads (off hand shooting) it doesn't matter. Also I find that for straight walled (revolver) cartridges, velocity variation from slight differences in powder charges gets totally swamped by powder placement in the cartridge at time of firing. So for my revolver rounds (most of what I reload) I use charges dropped straight from my measure. I do use a removable baffle in my Lyman measure. Its a quick easy way to increase consistentcy from a full to nearly empty powder reservoir.

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