I hope some of you physics types, or maybe some of you experienced shooters, can help
me figure out the physics of wind drift. I had a discussion with some folks at the range
the other day that left me confused.
I have access to two ranges--an indoor range with 25 yds. max, and an outdoor range that
is frequently windy. Very windy. So when I got my Rem 700 in .308, I sighted it in at
the indoor range at 25 yds (after dark on a work night), then hit the outdoor range
Saturday to sight in at 100yds.
It was blowing. The flags at the range were about straight out, with the wind exactly
crosswise to the range. Blowing from right to left. I estimate the wind at a steady 10-12 mph. I was shooting at a sight-in target, with 5 bullseyes. I was lucky I shot my first group at the upper right bullseye, ‘cause the group wound up near the upper left bullseye. That’s over a foot left of where I was aiming.
Now I didn’t want to sight my scope to the right bulleye, and then find out that, without that wind, I’d wind up a foot RIGHT of where I wanted to be when there was no wind. So I shot another group at the lower right bullseye, and that group wound up near the lower left bullseye. The groups were just over an inch, so I was happy, considering the wind. But when I asked the range boss how much I should expect the wind to move my point of impact, he said it should only move it about an inch. Now remember, I’d sighted in the night before at 25yds, and I found it hard to believe that I could be that far off, or that a wind that strong would only move my POI an inch.
Well, I did some math, to try to figure what that wind would do to POI. Here are my
calcs.
If the wind is 10 mph, I’ll convert to FPS to make the next step easier. 10mph times 5,280 for feet gives me 52,800 FPH (feet per hour)
Divide by 60 to get feet per minute gives me 880 FPM
Divide by 60 again to get FPS and I get 14.7FPS wind speed.
Now, how long is the bullet in flight, to be acted upon by that wind? My factory 165gr
round is rated at 2700fps muzzle, 2,500fps at 100yds. So the average velocity over the
whole trip is 2,600fps. So, to travel 300 feet at 2,600fps means 300/2600 or .115
seconds.
Winds at 14fps for .115 seconds should move the bullet 1.7 feet, or about 20.4 inches.
Right?
Well, the discussion that ensued between me, the range boss and several other shooters
got pretty lively. I will summarize.
One view: MOVING MEDIUM The bullet is traveling through a medium that is moving
(the air) so the air will move the bullet as much as it moves. i.e. 20.4 inches. That’s a bit more movement than I was seeing on the targets, but not a lot. Same as if a speedboat is crossing a current. The captain can predict his arrival point by calculating how much the current will move his boat in a specified time.
Another view: INERTIA. That bullet wants to move in a straight line. The wind wants
to move it. But the bullet’s inertia prevents the wind from moving it as much as the math would say, so the POI is closer to aim than the math would say. Hmmmm.
Makes some sense.
Another view: AERODYNAMICS. The more aerodynamic the round is, the less the
wind will affect it. An aerodynamic car moves less in a crosswind than a boxy car. So wind will move an aerodynamic bullet ( a boattail) less than it would move a boxy bullet. Or is that just the velocity making the time in the air shorter, so the bullet moves less?
Another view: WEIGHT. Some folks said a heavier bullet will move less than a light
bullet, because it has more inertia.
So who’s right? I favor the “moving medium” view, and think the math tells where the
bullet is going to hit. The other factors relate to velocity, which relates to time in the air, which relates to more or less movement. But nobody could agree on what factors were affecting the bullet how much.
Your thoughts, please?
me figure out the physics of wind drift. I had a discussion with some folks at the range
the other day that left me confused.
I have access to two ranges--an indoor range with 25 yds. max, and an outdoor range that
is frequently windy. Very windy. So when I got my Rem 700 in .308, I sighted it in at
the indoor range at 25 yds (after dark on a work night), then hit the outdoor range
Saturday to sight in at 100yds.
It was blowing. The flags at the range were about straight out, with the wind exactly
crosswise to the range. Blowing from right to left. I estimate the wind at a steady 10-12 mph. I was shooting at a sight-in target, with 5 bullseyes. I was lucky I shot my first group at the upper right bullseye, ‘cause the group wound up near the upper left bullseye. That’s over a foot left of where I was aiming.
Now I didn’t want to sight my scope to the right bulleye, and then find out that, without that wind, I’d wind up a foot RIGHT of where I wanted to be when there was no wind. So I shot another group at the lower right bullseye, and that group wound up near the lower left bullseye. The groups were just over an inch, so I was happy, considering the wind. But when I asked the range boss how much I should expect the wind to move my point of impact, he said it should only move it about an inch. Now remember, I’d sighted in the night before at 25yds, and I found it hard to believe that I could be that far off, or that a wind that strong would only move my POI an inch.
Well, I did some math, to try to figure what that wind would do to POI. Here are my
calcs.
If the wind is 10 mph, I’ll convert to FPS to make the next step easier. 10mph times 5,280 for feet gives me 52,800 FPH (feet per hour)
Divide by 60 to get feet per minute gives me 880 FPM
Divide by 60 again to get FPS and I get 14.7FPS wind speed.
Now, how long is the bullet in flight, to be acted upon by that wind? My factory 165gr
round is rated at 2700fps muzzle, 2,500fps at 100yds. So the average velocity over the
whole trip is 2,600fps. So, to travel 300 feet at 2,600fps means 300/2600 or .115
seconds.
Winds at 14fps for .115 seconds should move the bullet 1.7 feet, or about 20.4 inches.
Right?
Well, the discussion that ensued between me, the range boss and several other shooters
got pretty lively. I will summarize.
One view: MOVING MEDIUM The bullet is traveling through a medium that is moving
(the air) so the air will move the bullet as much as it moves. i.e. 20.4 inches. That’s a bit more movement than I was seeing on the targets, but not a lot. Same as if a speedboat is crossing a current. The captain can predict his arrival point by calculating how much the current will move his boat in a specified time.
Another view: INERTIA. That bullet wants to move in a straight line. The wind wants
to move it. But the bullet’s inertia prevents the wind from moving it as much as the math would say, so the POI is closer to aim than the math would say. Hmmmm.
Makes some sense.
Another view: AERODYNAMICS. The more aerodynamic the round is, the less the
wind will affect it. An aerodynamic car moves less in a crosswind than a boxy car. So wind will move an aerodynamic bullet ( a boattail) less than it would move a boxy bullet. Or is that just the velocity making the time in the air shorter, so the bullet moves less?
Another view: WEIGHT. Some folks said a heavier bullet will move less than a light
bullet, because it has more inertia.
So who’s right? I favor the “moving medium” view, and think the math tells where the
bullet is going to hit. The other factors relate to velocity, which relates to time in the air, which relates to more or less movement. But nobody could agree on what factors were affecting the bullet how much.
Your thoughts, please?