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Help! Extreme spread and standard deviation loads

 
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  #1  
Old 03-21-2011, 08:50 AM
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Help! Extreme spread and standard deviation loads

What affects extreme spread and standard deviation the most in a load, powder or length. Most of my 100 yd. accuracy loads have high ES and SD numbers, i.e. 80ES 36SD. I want to tune the loads so that they have low ES and SD numbers. Will overall length make a difference, or should I focus on how much powder I put in, or do I have to use a different powder.

I want to get my .308 win. below a 20fps ES.

Thank you.
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Old 03-21-2011, 02:09 PM
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Re: Help! Extreme spread and standard deviation loads

I address powder first, then seating depth, all the while paying attention to brass prep including case length.

I usually run a ladder test in .5g increments with the bullet seated .010" off the lands and try to find a couple of nodes with good accuracy while checking for signs of pressure.

After the first ladder test, I try some loads at smaller powder increments around one or two of the nodes that gave good accuracy. Around one of these nodes, the grouping and ES/SD should be less sensitive to +/- .1g of powder.

Then, I take the best of those results and start varying seating depth which will also affect results. Meanwhile, you also need to pay close attention to consistent case length and brass prep by this time. Lack of attention/consistency with any of those variables will affect your ES/SD for your MV.

Hope this helps.
-- richard
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Old 03-21-2011, 02:16 PM
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Re: Help! Extreme spread and standard deviation loads

As part of brass prep, I also try for consistent neck tension by turning necks to shave about 85% of the high spots and then use either bushing or collet dies to neck size fired brass where the ID is about .002" less than the bullet OD (before springback).
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Old 03-21-2011, 04:04 PM
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Re: Help! Extreme spread and standard deviation loads

Follow rscott's advice and you will do just fine.
What bullet and powder are you using.?
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Old 03-22-2011, 09:13 AM
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Re: Help! Extreme spread and standard deviation loads

Hornady 168 gr. A-MAX and Varget, over CCI BR2 primers. .308 Win.

Unfortunately this is a factory gun, and I can't use the internal box if I seat the load to the rifling. The jump is 0.108" before I can do anything but single load.
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Old 03-22-2011, 09:31 AM
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Re: Help! Extreme spread and standard deviation loads

I would try it as a single shot to see if that get's your accuracy/consistency where you want.

If so, you may be able to modify the magazine with one of the aftermarket products.
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Old 03-22-2011, 01:47 PM
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Re: Help! Extreme spread and standard deviation loads

Good advice here. How sure are you of your chrony's accuracy and what conditions were you shooting in?

-Mark
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