Which value for Pressure in our software???

I've found a discrepancy in the "standard" value for the Pressure, mainly, that affects VERY MUCH the calculatiosn for Long Range shooting.

As someone out there may be aware of some programs use as a default value 29.53 inches of mercury, (correspondig to the Army Standard Metro ) and others 29.92 inches of mercury, which corresponds to the ICAO standard metro.

In short, any clue of which one is the "right one"????

tks!

Posts: 21 | Location: Buenos Aires, Argentina | Registered: Tue November 09 2004

<KMB8>

Posted

Hi Gustavo,
Standard Metro is what most bullet mfgs use for figuring bullet BC's and is probably most useful.
IACO is more geared to Aviation.

A quick way of finding the data you need, assuming you have no ballistic programs, is to go to weather.com, plug in your zip code and find the barometric pressure for your shooting area.
The BP will usually be Std.Metro Sea Level which must be converted to your altitude.
As a general rule, each 1000' of altitude is worth a -1" in BP.
So if the BP at weather.com was 30.18 and your altitude is 3000' asl, then subtract 3" off.
That would be 30.18 - 3 = 27.18bp.

Most all ballistic software programs give the user the option of plugging in altitude, temp, RH and will calculate the correct BP, if you have unchecked "Use Standard Metro/Pressure etc....