# Muzzle velocity Deviation

#### daveyj

##### Member
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?

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)

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.

Fitch

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.

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