Help with MV variation

Kinjo

Kinjo

CGN Regular
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Location
southern Ontario
Did a ladder test for a new 260 using new Lapua brass achieving very consistent MV's. Then on to testing a couple of charges. Once again with new Lapua brass. Then test the same charges with once fired brass without resizing body, just resized the neck. Filled the same charge (double checked) and loaded my bullets. When I fired these cases my MV was all over the place with some dropping down as much as 250fps. So I figured that was my fault, I should resized the body and made sure the necks were bumped back the same.
So I resized my brass made sure the necks were bumped back the same amount. All brass weights the same, all charges double checked. But I'm still getting some unknown MV variations 2777, 2778, 2685, 2787. Three round with horizontal dispersion and the 2685 about 2" low @ 300yd.
The only thing I haven't done is neck turned. Anybody have the same problem, or can chime in I would like to hear your thoughts.

Thanks
 
Two variables come to mind.

Are you close to the rifling? If so, try seating the bullets 20 thou deeper.

Neck tension variations can cause a high ES. Try removing the expander button or the entire pin and neck sizing again, without the expander. This is max tension. One of my guns shoots best this way.
 
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I'll second Ganderite's comment- neck tension especially!

Another factor is case fill, as usually a better case fill will offer better consistency, but if you're testing the same charges then it's unlikely.

What about environmental? Similar temperatures and the like?
 
Ganderite said:
Two variables come to mind.

Are you close to the rifling? If so, try seating the bullets 20 thou deeper.

Neck tension variations can cause a high ES. Try removing the expander button or the entire pin and neck sizing again, without the expander. This is max tension. One of my guns shoots best this way.
Click to expand...

Bullet is seated ~ 15 thou, neck tension is the same from my collet die. Every bullet seat the same with a redding completion seater.
One other thing I did think of...I don't have the sun shades for my chrony. And a friend just txt me saying he had the same problem before with 100 - 250 FPS differences. Come to find it was the sun.
 
Consider this test... use another scale to confirm the charge weight and see if the problem continues.

If a group is stable and the chronie reports weird numbers, discount the chronie as light and cold do affect them.

When a shot goes low and the chronie says you are slow or high when the chronie says you are fast, odds are, the chronie is working just fine BUT something else isn't.

Since speed is directly related to the amount of fuel in your case, I would check your scale. Even a balance beam if that is all that is available. Chargemaster scales can tank and give false readings.

0.1gr of powder is 3 kernels of H4831SC... you aren't going to see any real difference in height and be off 1/2gr (15 to 17 kernels) of powder.

Last step, change the type or lot of your primer. Assume you have checked proper primer seating?

If all else fails, change the firing pin spring.

Jerry
 
I can confirm with out a doubt the whole problem was my chrony. I made a sunshade out of cardboard as I lost the ones that come with the unit. Placed it in the shade and fired 14 rounds through it. All MV's were very consistent with no major deviation.
Thanks for all your help guys.
 
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