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Muzzle velocity Deviation

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Unread 12-13-2009, 04:24 PM
Junior Member
Join Date: Jan 2006
Location: Northern MT
Posts: 23
Muzzle velocity Deviation

Does anyone have a good rule of thumb for computing fps deviation when the temps rise or fall? I know this varies based on what kind of powder we are using but I need a ballpark number.

When using exbal I cannot simply tell the program that I am zeroed at say 0 deg. I am zeroed at 60 deg with 2950 fps

If i tell it I am "rezeroed" (which I am not, I am a bit low) at 0deg I at least need to adjust the muzzle velocity correct?
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    Unread 12-13-2009, 05:20 PM
    Platinum Member
    Join Date: Oct 2007
    Location: Mountians of SW NC, near Asheville
    Posts: 1,595
    Re: Muzzle velocity Deviation

    Well, for an approximation, you could use the Universal Gas Laws to compute for the change in chamber pressure because of temperature.

    Multiply the peak chamber pressure in absolute (peak pressure in psi + 14.7) times the two Absolute Temps (add temp F to 460) for the change.

    T2 (absolute temp in the second instance)
    ___ =x Psi (absolute) = absolute pressure change

    T1 (absolute temp in the first instance)
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    Unread 12-14-2009, 09:17 AM
    Silver Member
    Join Date: Aug 2009
    Location: Carlisle, PA
    Posts: 478
    Re: Muzzle velocity Deviation

    I use QuickLoad to get an estimate of MV as a function of ambient temperature. It's different for every powder. It is also affected by how long the bullets have been "outdoors", if they soak in a hot chamber before being fired, and so on.

    I don't have a rule of thumb.

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    Unread 12-14-2009, 01:16 PM
    Platinum Member
    Join Date: Nov 2005
    Location: Texas
    Posts: 1,845
    Re: Muzzle velocity Deviation

    I shoot over the 35P in the summer and winter----would be shocked at what you see between 95 and 40 degrees even with the "Hodgen" powders that are supposed to be formulated for that.

    Got to shoot them in different conditions to know bottom line.
    The Truth Is Not Always Good For Business!!
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