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Is Wind Deflection Proportional to Time of Flight?

Most of us believe the the faster a bullet travels, the less time it spends in the air and therefore the less it is effected by wind. But is this always the case?

By Jeff Johnston


Myth: Wind deflection is always proportional to time of flight.


The Expert Explanation: (According to Mr. Julian Hatcher in the "NRA Firearms Fact Book," aka The Bullshooter's Bible):


Wind deflection [in .22 LR, and simliar velocity rounds] is not  proportional to time of flight. Instead, it is proportional to the amount of delay in the flight caused by air resistance. The 1145 f.p.s. standard-velocity .22 LR round takes .287 second to go 100 yards, but would take only .262 second to cover the same distance in a vacuum. The latter figure is easily found by dividing 300 ft. by the speed of the bullet (1145 fps), which would remain the same throughout its flight if it were in a vacuum. Thus the delay caused by air resistance is .025 second with the standard-velocity ammunition. The 1335 fps high-velocity ammo, which will take .259 second to cover 100 yards would take only .225 second in a vacuum. Thus, the delay for theis bullet is .035 second, or 37 percent greater than that of the standard-velocity .22. THe high-spped round, then, suffers about 37 percent more wind deflection than the standard velocity.


This remarkable result is due to the very rapid rate at which air resistance increases with increase in bullet speed in the region near the speed of sound. The .22 rimfires are the only important rifle cartridges that occupy this speed range, and they are the only ones that show more wind deflection as velocity is increased. While air resistance always increases when the bullet is shot faster, the rate of this increase is less steep at supersonic velocities. [Supersonic] rifle bullets in general, contrary to the case of rimfires, are made less sensitive to the wind by raising their velocities.


The Conclusion: Sometimes, but not always. In cartridges that travel at speeds near the speed of sound (like the .22 LR), wind deflection is not proportional to time of flight. So the slower a .22 round is, in general, the less it is deflected by wind. In supersonic rounds, however, wind deflection is proportional to time of flight, so the speedier the round, the less it is deflected by wind. This is why competitive small bore shooters choose standare velocity .22 rounds over high-velocity rounds.


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3 Responses to Is Wind Deflection Proportional to Time of Flight?

William Fisher wrote:
December 14, 2013

Please edit my post to read- Bullet Lag Time is the difference between the Actual TOF and the TOF in a vacuum (Range divided by Muzzle Velocity).

William Fisher wrote:
December 14, 2013

This statement is incorrect- 'In supersonic rounds, however, wind deflection is proportional to time of flight,' The wind deflection is actually proportional to the bullet 'Lag Time', which is the actual Time of Flight minus the Muzzle Velocity.

Allan Waggoner wrote:
September 21, 2011

So that's why the 6.5 Lapua & .260 Rem get less wind drift than the .308.