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  #1  
Old 07-25-2011, 01:10 AM
mattri mattri is offline
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Default First time using chrony for 20 Practical- strange results

Just got a new chronograph, its a Competition Electronic's ProChrono Digital.

Went to the range today and had some strange results.

20 Practical, 5 shots, 1st: 3820fps, 2nd: 3875fps, 3rd: 3910fps, 4th: 3887fps, 5th: 3875fps. Here's the group:




Another similar group, this one was only three shots. 1st shot: 3910fps, 2nd shot: 3853fps, 3rd shot: 3933fps.




I've noticed some inconsistent neck tension while seating bullets. I assume this has something to do with the spread in velocity? All loads are metered very, very carefully and COL checked with a micrometer mounted comparator.

It would seem that such a big spread in velocity would have a more direct affect on accuracy- any suggestions?

Interstingly, while at the range I also measured some loads for my 30-30s. In two different rifles, after almost 20 rounds all the shots were within 40fps of each other.
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Old 07-25-2011, 03:00 AM
5mmgunguy 5mmgunguy is offline
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Seems a little high to me. Most of mine are about 50 fps absolute spread. Best I have is a neck turned ppc and get about 15 -17 absolute spread.
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Old 07-25-2011, 04:11 AM
montdoug montdoug is offline
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The large deviation in velocity will show up in your groups a lot more at further distances.
If you have a ballistics program on your computer figure that information using 3,820 FPS and then with all else the same change the velocity to 3,910Fps. At 100 yards there is less than a 10th of an inch difference in holdover with a 200 yard zero which isn't a long distance in a 100 yard group (.3's to .4's etc) but at 400 yards there is almost 9ths of an inch difference in drop. Out at long distance those large standard deviations matter but still not much on a ground hog for all the tail chasing some do for max velocity.
Still, those kinda spreads send a guy back to the drawing board for sure. Your neck tension theory sounds like a first thing to eliminate in your search for a more consistent load.
On the positive side it looks like the rifle and you are both doing your parts, good shooting .
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Old 07-25-2011, 04:39 AM
ab_bentley ab_bentley is offline
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What powder/ primer are you using? Also bullet weight? Adam
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Old 07-25-2011, 11:51 AM
mattri mattri is offline
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23.1grs of H4198 in Win brass CCI 450s Sierra 39gr BKs.

The most common suggestion re the neck tension was to remove the expander button from the decapping rod of the Redding type S bushing die I use while sizing.

I had some rounds already loaded that had been sized with the button on, and they had a variance like the ones posted above.

I also ran some cases through the bushing die without the expander and loaded them. They had the same kind of spread in velocity.
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Old 07-25-2011, 12:23 PM
samspade samspade is offline
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The proof in the load is at the target and your group's look good??
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Old 07-25-2011, 08:17 PM
mattri mattri is offline
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Thanks, the gun has always been accurate, just confused as to what could be causing such a spread. When I say the loads were metered carefully I mean I measure each on on a scale, adding and removing individual grains of powder to get them exactly the same.
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Old 07-25-2011, 10:58 PM
TinMan TinMan is offline
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Your 5 shot group gave 33fps for SD and the 3 shot group was 41 fps for SD. I always use 5 shot group minimums. Decent load prep should give you under 20fps SD for a decent varmint/hunting rifle and under 10fps SD for BR type quality loads. The groups look pretty good, assuming at 100yds.
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Old 07-25-2011, 11:47 PM
Joe O Joe O is offline
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Try sorting your cases by weight,and weigh the bullets,as well as measure and sort with a comparator.set seating depth just off the lands.If you are getting different bullet tension,when seating bullets,consider turning to a uniform neck thickness,and use a bushing that is .002 less than a loaded round.OR,forget all that, and be happy with the great groups, you already have.
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  #10  
Old 07-25-2011, 11:49 PM
mattri mattri is offline
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Sorry to not be clear, all rounds are measured with a comparator.
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