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Old December 28, 2005, 10:52 PM   #1
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300 WSM and Standard Deviation

Howdy All, Just got back from the range and shot my CZ model 3 in 300 WSM for the first time today. I loaded 180 Grain Hornady Interlock SP BT to a average velocity of 2786 fps. 60 Grains of IMR-4350. It shot to about .75 inch with 3 rounds and about 1.05 inch with 6 rounds. I had a Standard Deviation of about 22. I have just started with the Standard Deviation calculations and do not know if that is good, bad, or average. Now I did read somewhere that trust the paper, not the numbers. Any words of wisdom out there?

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Old December 28, 2005, 11:23 PM   #2
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SD, uniformity, and other devils

Bogartbill31--SD is a statistical way of saying what the "normal" difference is from the exact average, in a group of numerical samples (like rifle bullet velocities.) The smaller the SD is, the better. I understand that an SD in the teens is very good, and an SD in the single digits is so good it's probably a mistake. So your SD of 22 certainly isn't bad. This is for how many shots, though? If only for a few, like 3 or 5 shots, it doesn't tell you much. Statisticians call that too small a sample. A 10-shot group will tell you a lot more about how uniform the velocities are for your load.

The whole idea (as I'm sure you know) is to keep things as uniform as possible from shot to shot, therefore keeping the POI for each shot as close to all the other POI's as possible. So you treat the cases all the same. Carefully measure the powder--probably weigh each charge with a stick powder like IMR 4350 (one of my own favorite rifle powders, BTW!) Hand-seat the primers. Do as many of the putzy reloading procedures as you can, but do them the same to each and every round!

But you're right, the proof of the pudding is in this case the group on the paper. Just offhand, I'd say you should be able to get smaller than a 1-inch group at 100 yds with 5 or even 10 shots, from a really good solid rest, with a quality .300 WSM, using handloads it likes. And I betcha if you do, yr SD will be smaller, too!
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--Smokey Joe
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Old December 29, 2005, 12:38 PM   #3
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You are from 5 to 9 grains under max,(depending on which manual you look at). Given that fact, your powder position is one reason for higher SD #'s and larger group size. Most rifles like a higher percentage of capacity powder charge,(above 95%). It can't be out of position in relation to the primer flame. At 60.0 grains, you are close to 10% under max. You should NOT go UNDER that!

You should be able to get to, or real close to, 3000 fps with the 300 WSM. I can easily get there with numerous loads in my 23 inch Browning a-bolt.
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Old December 29, 2005, 02:26 PM   #4
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snuffy made some good points.

I've also seen higher SD numbers when using a powder that doesn't flow smoothly through powder measures. The old long-grain, extruded IMR powders are particularly bad. I still have some of them on the shelves in the reloading room.
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