Need help with chrono results!

Discussion in 'Reloading' started by dodgefreak8, Oct 9, 2007.

  1. dodgefreak8

    dodgefreak8 Well-Known Member

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    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?
     
  2. britz

    britz Well-Known Member

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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.
     

  3. ss7mm

    ss7mm Writers Guild

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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?
     
  4. dodgefreak8

    dodgefreak8 Well-Known Member

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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.
     
  5. AJ Peacock

    AJ Peacock Well-Known Member

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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
     
  6. J E Custom

    J E Custom Well-Known Member

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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
    J E CUSTOM
     
  7. LewisH

    LewisH Well-Known Member

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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: Oct 10, 2007
  8. dodgefreak8

    dodgefreak8 Well-Known Member

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    this load has shot well before though?? with the same batch of powder/primer combo. I have shot H100 as well and I might just go back to that since it's an extreme powder. Last time I shot this load it was 32 degrees. That is the only changing variable I can think of. Thing is the rifle shot fairly decent groups. about 1.25" at 100 yrds. I just feel there is room for improvment on my grouping since my velocities are so different. Also to answer a couple more questions. This brass is on it's 3rd firing and the chrono was about 12' (4 steps)away from the muzzle.
     
  9. HRstretch

    HRstretch Well-Known Member

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    I know that down here in Houston the higher temps will cause variances in velocities and pressures. I have had changes in max loads that shot fine in mid 80's temperatures and then had pressure problems when temperatures went into the 90's. This was using IMR 7828, I am going to try H1000 to see if i can achieve maximum group and velocity conditions. My biggest concern is that I an going to have to change my load when the temps drop?
     
  10. goodgrouper

    goodgrouper Well-Known Member

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    I normally have to have a summer and a winter load with ANY powder regardless of "extreme' or not. And it seems that when temps are nearing 80 degrees, all sorts of weird pressure irregularities can occur with pre-loaded ammo. Just how long you leave a loaded round in your chamber when the barrel is warm and the air temp is warming up can affect your combustion. And there ain't no magical powder coating or marketing hype that can change that.

    Also, the list the AJ provided is good to look at. I would also add that cases that are different in headspace length due to dies that aren't really full length sizing can change pressures too.

    And there are chronos that don't work as well as others and could be affected by muzzle blast, partly cloudy days, weak batteries, and so on.