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Velocity inconsistent

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  #1  
Unread 07-25-2012, 06:34 AM
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Velocity inconsistent

I’m testing the berger hunting vld 168 gr bullets in my 7mm stw. The speeds I’m getting vary from 3,240 to 3,285. This load shots great but I’m worried about the inconsonant velocities. I’m new to chronographing loads, so is having a load plus or minus 45 fps reasonable??? If not, what should I expect the variance to be?

Last edited by smokeumm; 07-25-2012 at 07:13 AM.
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  •   #2  
    Unread 07-25-2012, 08:28 AM
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    Re: Velocity inconsistent

    need a little more info to help come up with answer, how far from muzzle is crono? how many shots to average? what crono? battery new in crono? what is vertical spread on target and what range to target?
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      #3  
    Unread 07-25-2012, 08:45 AM
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    Re: Velocity inconsistent

    The chrono is 10ft from muzzle. I've shot ruffly 10 shots. The battery is a few months old, but the chrono has maybe been used 5 times since I bought it. It is a Competition Electronics ProChrono Digital Chronograph. The range is 100 yards and my grouping is good, maybe 1/2 of an inch,, no more. The vertical spread is around 3/8 of an inch.

    If everything is dialed in with a load, what is a reasonable variance in speed?
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      #4  
    Unread 07-25-2012, 08:55 AM
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    Re: Velocity inconsistent

    move crono to 15' from muzzle, and make sure it's level. also, use sky screens all the time.
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      #5  
    Unread 07-25-2012, 09:03 AM
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    Re: Velocity inconsistent

    Quote:
    Originally Posted by MTBULLET View Post
    move crono to 15' from muzzle, and make sure it's level. also, use sky screens all the time.

    AGREE with the above. At 10' from the muzzle you are possible getting some false readings due to muzzle blast. In fact I put mine at 20'. Keep in mind that at that distance you need to go back into a balllistic program to get "true" muzzle velocity which I would guess will be about 10-15 fps faster than your chrony is reading.
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      #6  
    Unread 07-25-2012, 04:18 PM
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    Re: Velocity inconsistent

    The news is out... velocity will vary a bit from shot to shot.....

    A bit of variance is normal. You should try to stay away from 100 fps extreme deviation by a long shot, but a 20 fps or so x like you are talking about isn't that bad. Like the others have said, get the chrono back a few feet(15-20) and level, use diffusers or sky screens, and then you'll know what your real variation is.

    Btw, 3263 fps average is 100 fps or more hot for an stw with a 168, if you drop your charge to a more reasonable level, your x's may come down some. At the least, you'll be saving bore and action life and getting more reloads out of your brass. General rule with an stw is 3400 with 140's, 3300 with 150's, 3150-3200 with 160's, and 3000 with 175's. Your 168 will be at 3100 fps at normal top pressure with a 24-26" barrel. I've worked with a bunch of these rifles, and none has been uniform well over pressure.
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      #7  
    Unread 07-29-2012, 10:33 AM
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    Re: Velocity inconsistent

    Thanks for all your help and ideas? I will try 20 feet with a screen!

    I thought the speed I'm getting was quite high myself. This is a custom rifle made by McWhorter. The load data supplied by McWhorter is 83 grs Vihtavouri N570 powder, with 168 grs vld berger. The last thing I want to do is to damage this gun. If this load is unsafe for the rifle I will for sure tame it down???

    This gun shoots excelent, but I have not tested it for long range shoting. I'm in the process of learning how it shots first.
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