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Need help with chrono results!

 
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  #1  
Old 10-09-2007, 07:53 PM
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Need help with chrono results!

I took my 7mm STW out today to get ready for my deer season and I have some strange chronograph results. In 9 shots my velocity ranged from 3137fps to 3292fps. . The load is 78gr of RL-22 and a nolser 160gr accubond with fed 215M primers I don't understand how I am getting this large of a spread. Also I am loading 20 thou. off the lands. I full length resize and weigh all my loads on an RCBS digital scale. I also use a flash hole uniformer. Are these normal spreads or is something going on hear I'm not catching?
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Old 10-09-2007, 08:48 PM
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I for one have not had good luck with a chronograph. I was shooting targets at 700 yards, hitting a 5 inch vertical spread with chrono readings that were varying over 120 fps. I don't think that I would have been hitting that well if the loads were that bad. Some days my chrono gets consistant readings and others it doesn't. the best weather is a overcast sky for the chrono.
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Old 10-09-2007, 09:58 PM
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Have you chronographed them before and if so, what were they then?

What was the temperature when you were shooting the 9 rounds?

How fast were the shots fired?

Is there a chance the barrel/chamber was getting hot and you let the loads cook in the chamber before shooting them? It makes a lot of difference if you let them cook in a hot chamber before firing them.

What brand of chronograph?
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Old 10-09-2007, 10:13 PM
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chrono is a prochrono digital and they usually chrono around 3150 fps. Temp was about 75 dregrees and I fired them in 3 shot groups waiting 1 min between each round with a 10 minute interval between groups. I chambered each round right before firing it.
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Old 10-10-2007, 08:39 AM
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4 long shot/ SWAG's (silly wild a** guesses)

1) I've seen differences in neck tension cause that much variation. You might measure the neck thicknesses and see if they are all consistent (its a long shot). I've seen it when comparing different bushings or dies with the same load.

2) Over-Uniforming (enlarging) of the flash holes can also lead to velocity spreads. (another long shot)

3) How close was the chrono to your muzzle? I've had 'silly' readings when the chrono was too close (I think it was due to muzzle blast).

4) Had all the brass been fired the same number of times? Some new, some once fired?

Hope one of these helps,

AJ
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Old 10-10-2007, 10:31 AM
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Dodge

I had the same problem with my 7 STW an tried a lot of
things an this is what fixed my velocity spreads and S D's.

The STW is a large over bore case but not so large that
it "requires" 215 mag primers to ignite uniformly even
when using very slow powder like H5010.

So I switched to Rem 9 1/2 mag and this was the results.

In a custom 31" barreled rifle with 140gr blis tip and H 5010
fed 215 primer velocity was 3569 ft/sec and S D was 23 ft/sec
With a rem 9 1/2 mag primer velocity was 3537 ft/sec and
S D was 11.

I have found that in cases that hold more than 95 grs a 215
is almost allways required. below that depending on bullet
weight and load density a 9 1/2 will work even better most
of the time, Some velocity loss is normal (25 to 50 ft/sec)
but Standard Deviations normaly are cut in half improving
accurace.

If your are doing all the things that have been suggested then
try the primer change.

Just another option
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  #7  
Old 10-10-2007, 05:10 PM
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Lots of RL22 and 25 have reportedly been very inconsistent in the last few years, so much so that some reloaders have abandoned their use altogether.

You might try Retumbo or H1000.

Last edited by LewisH; 10-10-2007 at 05:30 PM.
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