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God groups but bad ES

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  #1  
Unread 03-15-2011, 02:32 AM
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Join Date: Jul 2009
Posts: 61
God groups but bad ES

I have a custom built 300 RUM that I had built two years ago that It shoots sub 1/4" MOA from 100 to 700 yards (3 shot groups); however, when I use my chrono, it shows an ES of about 25 fps. Even if I throw out the most extreme velocities, it would still show an ES of 20 fps. What can be causing this?

When I shoot at 750 yards, the groups tend to be slightly more horizontal than vertical.

Here are the specs of my rifle.

Stiller Predator Action
Schneider P5 barrel, 1:11 twist -
5.5 taper, 28"+ Muzzle Break
McMillan A 3-5 Stock
5.5x22x56 Night Force
Badger Rings

Nosler Brass
86 grains of H1,000
Federal 215 M Primers
Berger 210 Hunting VLDs
loaded .020 off the lands
OAL is 3.798 (4.127 with comparator)
2,988 to 3,013 FPS

I measure every load exactly on an older RCBS Balance Beam scale
RCBS rock chucker press
RCBS full length dies and seater

Just as an FYI, I have no problem achieving a low ES when I reload for my friends 300 RUM (6- ES), or my Kimber 300WSM (9- ES); however these guns only average .75" group at 100 yards.

The other thing that is throwing me off is when I last shot at 750 yards; I shot 3 groups that were (1.5") (3.2") (2.5"). When I do the math on my Ipod (ballistic FTE), and figure a 25 fps ES, my bullets should be printing in a 6" vertical group.

I know I should be happy with what I'm getting, but I can't help but think what my 1,000 yard groups would be like if I had an ES of 5?

Where should I begin my search for th cure?

Justin
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  •   #2  
    Unread 03-15-2011, 07:30 AM
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    Join Date: Mar 2011
    Location: N. E. Montana
    Posts: 44
    Re: God groups but bad ES

    2.5" groups at 750?.....mission acclomplished. Throw away the chronograph.
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      #3  
    Unread 03-15-2011, 07:45 AM
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    Join Date: Nov 2009
    Posts: 90
    Re: God groups but bad ES

    ^^I agree. But you could try a few loads with some different powder/ seating depth. Shoot groups at 750 without the chrono, and pay attention to vertical spread. What type of vertical spread do the other rifles you load for get?

    This is what i do, and my current rifle prints groups with almost no vertical dispersion (1/2") @600yds....
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      #4  
    Unread 03-15-2011, 07:51 AM
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    Join Date: Feb 2007
    Location: Townsend, Montana.
    Posts: 8,548
    Re: God groups but bad ES

    Move your chronograph farther away from the muzzle and retest. 18' plus.

    Jeff
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      #5  
    Unread 03-15-2011, 09:21 AM
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    Join Date: Nov 2005
    Location: Texas
    Posts: 1,845
    Re: God groups but bad ES

    25 fps is actually marginally acceptable for a sporter. My cut off for competition loads is 15fps. The only thing I question is your shot string. When verifying the final load I always shoot a couple of 10 shot strings to verify my ES.
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      #6  
    Unread 03-15-2011, 09:40 AM
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    Join Date: Jun 2007
    Location: Burlington WY
    Posts: 98
    Re: God groups but bad ES

    I would have to say groups don't lie. Those are good groups for that large of an ES, I would personally question the chrono. Was it a partly cloudy day? I found that most chronographs are very sensitive to light conditions and where the sun is in the sky.
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      #7  
    Unread 03-15-2011, 09:45 AM
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    Join Date: Jul 2007
    Location: Mt., Id. , SD
    Posts: 1,049
    Re: God groups but bad ES

    Quote:
    Originally Posted by Broz View Post
    Move your chronograph farther away from the muzzle and retest. 18' plus.
    Jeff
    Broz gives good advise.

    I think what Broz is saying is that you have to be assured you are not getting any effect from muzzle blast on your chrony.

    Heck - set your chrony up at 100 yds if your rifle is shooting like that and then you'll really know what is happening. Use your 100 yd. velocities and play with a ballistic calculator to get your muzzle velocity.
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