B
benenglish
Guest
...because this isn't the case.
With respect - no.
...it's probably simpler to use conditions that avoid transonic flight...
The exception I'm aware of ... has to do with increased drag in the transonic range ...
Right.
Which is exactly the range (850 fps vs 1700 fps) that I was talking about. The exception doesn't stop (ie, wind drift doesn't get back to 850 fps levels) until velocity increases to about 2300 fps.
I'm a pistol silhouette shooter. This is the range in which I live. BPCR shooters have to deal with it, too.
When I first stumbled on this it seemed quite counter-intuitive so I wrote Sierra and asked for some explanation why their ballistics software was showing less wind drift for the same bullet over the same distance in the same conditions when it was launched at 900 fps than when it left the barrel at 1400. Here's a part of the response I got from Dr. Bill McDonald:
You have discovered a phenomenon which is real and which affects both handguns and rifles firing bullets at transonic and subsonic velocities. We first ran into the same effect several years ago when we were working with bullets from a .45-70 rifle fired at velocities ranging from about 1200 to 2000 fps. I could not believe the wind deflection numbers either until I performed a considerable amount of analysis of the physics involved. I analyzed the situation using the closed form ballistic solutions that we published in the second and later editions of Sierra’s Reloading Manuals. To shorten a very long story, the apparent anomaly in wind deflection occurs because the air drag on a bullet decreases so radically when velocity falls through the lower transonic and upper subsonic velocity regions.
At muzzle velocities above about 1500 fps, wind deflection decreases as muzzle velocity rises because lag time (difference between actual time of flight and the time of flight the bullet would have in a vacuum) decreases as the bullet travels faster between muzzle and target. So, as a result maximum wind deflection occurs for a muzzle velocity somewhere in the 1300 to 1500 fps region. At lower muzzle velocities the air drag on the bullet decreases very rapidly, so that the bullet is not slowed down nearly so much by drag. Then, the actual time of flight begins to approach the vacuum time of flight, and wind deflection then decreases at these lower muzzle velocities.
He then went on to talk about still lower velocities as seen in air guns and to spend quite a bit of time taking me to task for some very sloppy arithmetic I had included in my original email to Sierra.
So, yeah, a bullet launched at 900 fps will drift less than one launched at 1700.
I see, after doing some reading, that this does NOT contradict the Didion approximation you referenced.
Learning stuff is fun. Seriously.