Help! Extreme spread and standard deviation loads

[bowhunthard88]

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.

 

[rscott5028]

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

 

[rscott5028]

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).

 

[RockZ]

Follow rscott's advice and you will do just fine.
What bullet and powder are you using.?

 

[bowhunthard88]

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.

 

[rscott5028]

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.

 

[MontanaRifleman]

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

-Mark

 

[bowhunthard88]

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

-Mark

I have a Shooting Chrony. About 45 degrees F, semi-cloudy conditions.

 

[MontanaRifleman]

I have a Shooting Chrony. About 45 degrees F, semi-cloudy conditions.

I have a shooting chrony too, but I don't trust it for accurate ES's. I think it's overall accuracy is fairly close but I'm sure it has a margin of error. A +/- 1/2% margin of error for 3000 fps amounts to 30 fps possible error. If you can compare with a couple of other chronys of different makes, that might give you a good idea of accurate it is. I only put so much faith in mass produced instruments that cost about $100.

If your light conditions varied at all, then it very likely affected the readouts. I've seen shifts of up to 100 fps as the sun goes in and out of clouds. Other factors to consider are how far is the chrony from the muzzle and how is the battery charge?

I judge my loads by actual performance.

 

[rscott5028]

If you doubt the chrony, then shoot some long range groups.

If you have low vertical dispersion in your long range groups, your ES/SD is probably ok.

Otherwise, you may have issues with ES/SD, equipment, or technique.

Groups don't lie. But, they don't always tell the whole truth either.

-- richard

 

[bowhunthard88]

If you doubt the chrony, then shoot some long range groups.

If you have low vertical dispersion in your long range groups, your ES/SD is probably ok.

Otherwise, you may have issues with ES/SD, equipment, or technique.

Groups don't lie. But, they don't always tell the whole truth either.

-- richard

The furthest I've shot it so far was 350 yds., the groups averaged from 0.5" - just under an inch.

 

[rscott5028]

The furthest I've shot it so far was 350 yds., the groups averaged from 0.5" - just under an inch.

That sounds pretty respectable to me. So, I believe I'd temporarily ignore the chrony and try shooting farther.

-- richard

 

[bowhunthard88]

Guess I'll look for someplace I can shoot farther lol.

 

[Yoshie]

I have had some problems with my shooting chrony also. its extremely important to have it 10 or so feet from the muzzle and pretty much new batteries.