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Old 8th September 2011, 08:10 PM
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villiers villiers is offline
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Join Date: Nov 2008
Member of: Bisley
Location: Uxbridge UB8
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Default Air pressure v air density

A question related to last weekend shooting at the worlds. How much effect does air pressure ,air density and air temp have on the flight of pellets. On the zero range I started out two 1/4 click high at 55 yards, I had a number of pellets split at the top of the kill at 35 yards and I felt I was in a good position and happy with the shot I don't believe I was that much out with my target distance so I had a pellet rise 20mm is this possible ? . This happened just before a rain storm did this have any bearing on the miss. I did check first thing in the morning a number of dialing numbers at known ranges which did seem to line up with what I normally see. Have cronoed the gun and all is where it should be.
The top of my head is like a baboons bottom from all the head scratching.

Water vapor
The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear contrary to logic.
This occurs because the molecular mass of water (18 g/mol) is less than the molecular mass of dry air (around 29 g/mol). For any gas, at a given temperature and pressure, the number of molecules present is constant for a particular volume (see Avogadro's Law). So when water molecules (vapor) are added to a given volume of air, the dry air molecules must decrease by the same number, to keep the pressure or temperature from increasing. Hence the mass per unit volume of the gas (its density) decreases.
The density of humid air may be calculated as a mixture of ideal gases. In this case, the partial pressure of water vapor is known as the vapor pressure. Using this method, error in the density calculation is less than 0.2% in the range of −10 C to 50 C.
As barometric pressure decreases, the bullet's path is flatter, requiring fewer come-ups on your elevation dial. As temperature decreases, the bullet's path may be less flat and the muzzle velocity reduced. Reticle designers and theorists David Tubb and Brand Cole have included density/altitude charts within the image field of their scopes to help you figure this out. However, the chart is dependent on bullet configuration and muzzle velocity to arrive at the necessary come-up changes. They have done this for incline shots and devised elegant rangefinding techniques within the scope's image field as wel
Gone racing

Last edited by villiers; 8th September 2011 at 08:39 PM.
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