What’s The Best Velocity For Slugs To Minimize Wind Drift?
Back in July 2029, HAM published an article by Technical Editor Bob Sterne about the best velocity for slugs to minimize wind drift. You can see it at: https://hardairmagazine.com/ham-columns/the-external-ballistics-of-slugs-in-airguns/
Recently a HAM reader contacted the HAM offices with a question for Bob based on this post. First he highlighted the section that concerned him, then moved to a question…
“Note that the least wind drift occurs when starting with a muzzle velocity of about 900 fps, and increases when the MV is either slower or faster. The wind drift at an MV of 1500 fps is about 50% more than at 900 fps. In fact it is as poor at 1500 fps as it is at about 600 fps for most slugs we use. You have to use an MV of about 2500 fps before the wind drift is as low as what it is at 900 fps!
This chart is quite a shock to most slug shooters. It shows an increase in drift of about 10% by increasing the MV from 1000 fps to 1100, and an even greater increase above that. Combined with how much more air it takes to reach 1100 fps, if shows that even for slugs there is little point striving for much over 1000 fps.
” Are you sure about this?” wrote our reader. “I’m pretty sure the slower a projectile is moving the MORE wind drift there will be, at any velocity.” he continued.
Bob’s reply is very interesting…
Thanks for your question about the best velocity for slugs to minimize wind drift. It is a popular misconception that higher velocity reduces the wind drift, because it reduces the time of flight to the target.
Wind drift is caused by a sideways component to the drag force on the bullet (aka slug). Any projectile turns into the “apparent wind” (which is the vector sum of the forward and crosswind velocities), and hence actually experiences no “crosswind” per se.
Since the projectile is yawed into the crosswind, the drag force has a sideways component that causes the downwind drift.
The drag on any projectile increases with velocity, and experiences a drastic increase in the Transonic range (Mach 0.8-1.2). Each projectile has a different “drag curve”, but here is the one for the G1 profile, from with the BC(G1) is derived. Also shown are a sphere, and the boattailed G7 drag model.
Note the huge increase in drag between Mach 0.6 and 1.3 for the G1 model. The other profiles have that increase in a different speed range, but in every case it is the steepest at the speed of sound (Mach 1).
Wind drift may be calculated by finding what is caller the “lag time”. This is the difference between the actual time of flight from muzzle to target, and what it would be in a vacuum (with no drag). The latter is simply D / MV (distance over muzzle velocity).
As an example, at 900 fps over 100 yards (300 ft) the time of flight in a vacuum is (300 / 900) = 0.3333 seconds. The higher the BC, the less velocity is lost, so the smaller the lag time.
Here are the actual times of flight, and lag times, for a projectile starting at 900 fps, over that 100 yards, for various BCs:
BC = 0.1 …. 0.3595 sec…. Lag time = 0.0262 sec.
BC = 0.2 …. 0.3465 sec…. Lag time = 0.0132 sec.
BC = 0.4 …. 0.3399 sec…. Lag time = 0.0066 sec.
You can see that as the BC doubles, the lag time is half. This is why wind drift is proportional to BC, if velocity is constant.
If instead we choose a fixed BC, let’s use BC(G1) = 0.2, and vary the velocity, we can see the effect that has on the lag time, and the Drift (in inches) in a 10 mph crosswind at 100 yards.
450 fps …. 0.6890 sec…. minus 0.6667 (in a vacuum)…. Lag time = 0.223 sec…. Drift = 1.96”
900 fps …. 0.3465 sec. – 0.3333 sec….. Lag time = 0.0132 sec…. Drift = 1.16”
1350 fps…. 0.2445 sec. – 0.2222 sec…. Lag time = 0.0223 sec…. Drift = 1.96”
1800 fps…. 0.1849 sec. – 0.1667 sec…. Lag time = 0.0182 sec…. Drift = 1.60”
2250 fps…. 0.1468 sec. – 0.1333 sec…. Lag time = 0.0135 sec…. Drift = 1.18”
2700 fps…. 0.1214 sec. – 0.1111 sec…. Lag time = 0.0103 sec…. Drift = 0.90”
The above were calculated using the MERO Ballistics Calculator and the G1 Drag model, but you will get the same results using any calculator you choose.
You will note two important comparisons. In bold red, you will note that the drift at 100 yards is the same at 1350 fps as at 450 fps (3 times the velocity).
In bold black you will notice that the drift at 2250 fps is still greater than that at 900 fps. In fact, the minimum drift over 100 yards, in a 10 mph crosswind, with a BC(G1) of 0.20, occurs between 820-850 fps, at 1.13”. That’s the best velocity for slugs to minimize wind drift.
I know this is counter-intuitive, but Ballistics is a well established science, though often poorly understood.
It is the drastic increase in drag in the Transonic region that causes this effect. We are very fortunate in airguns to be able to shoot in the exact range of velocities where wind drift is minimized (since 2500 fps is not practical for us).
I hope this answers your question about the best velocity for slugs to minimize wind drift.
Bob, Thanks as always!