I'm still playing with the math trying to figure it out.
I've got all sorts of theories, but no proof. I do have a list of links
put together that I keep rereading in my attempt to minimize my ignorance.
Super Subsonics | Rifles | Guns & Ammo
My personal opinion is that at supersonic speeds, the air molecules are compressed
into a skin, or sail, around and in front of the supersonic round.
That increases the apparent cross sectional area of the bullet. That larger area
is more affected by the crosswinds than the slower ammo. The mass of a 40 grain
round nose remains the same from the moment it leaves the muzzle until it impacts.
The velocity changes from muzzle to impact due to atmosperic friction, with the hi-v ammo
slowing at a faster rate. Both rounds have the same shape, so the only thing left
that explains the difference, is the sail/skin created by supersonic velocity.
That's my theory, seems to fit my math, but I'm no physicist...
My other premise has to do with the number of molecular impacts that occur
in the time it takes to get from the muzzle to the target. At subsonic speeds
the space between air molecules and the speed that they move at, means the subsonic
ammo slides through the gaps. But the supersonic ammo impacts and deflects more
of the air molecules, thereby transferring the lateral energy of the molecules
to the bullet, causing increased drift. Like I said, just theories...;^)
Think of it as shooting in the rain. At slower speeds you hit fewer drops
so you get fewer impacts causing less deflection. At high speeds,
the number of impacts increase, thereby causing a larger deflection.
