Wind Drift for Slugs – We Need to Replace the G1 Drag Model
The G1 drag model is used by manufacturers to indicate downrange performance when shooting slugs. The BCs shown on all slug packaging today are calculated that way.
But is it a valid model to use? And does it give shooters the correct information they need? HAM Technical Editor Bob Sterne gives us his take…
The recent mini-article published in Hard Air Magazine https://hardairmagazine.com/ham-columns/whats-the-best-velocity-for-slugs-to-minimize-wind-drift/ was actually my reply to a question by a reader. I based that reply on the assumption that the G1 Drag Model was used, and that the slug was a reasonably close match to it.
Recent testing here in HAM https://hardairmagazine.com/ham-columns/the-big-ham-slugs-test-part-one-theres-no-single-bc-value-for-slugs/ found that the Ballistics Coefficient (BC) of most slugs increased with velocity, if the G1 model was used to calculate it.
This means that the G1 model is a poor match for typical airgun slugs.
I have mentioned this before in my article from Feb. 15, 2021, published in HAM https://hardairmagazine.com/ham-columns/some-new-airgun-slug-drag-models-by-miles-morris/
This mismatch between the G1 model and the actual performance of slugs in airguns continues to cause problems for us. Not only does the trajectory, downrange performance, and wind drift not match what the G1 model predicts, this mismatch leads us to incorrect conclusions (such as in the simplified answer I gave our reader).
There are many different Drag Laws, and for a given slug, each one will produce not only a different BC, but a different optimum muzzle velocity (MV) to obtain minimum wind drift. Here are some of the commonly used drag models:
In airguns, we are only interested in the Subsonic and Transonic regions (below Mach 1.2), so here is that portion enlarged for clarity. Note the sudden increase in drag around the Speed of Sound (~1126 FPS). This causes a correspondingly large increase in the wind drift, but it is not the same for all projectiles:
The BC Depends on the Drag Model Chosen.
The most accurate way to determine the Drag Coefficient (Cd) of a projectile is to have an accurate MV, and to know the time of flight to the target (to within 1 mSec).
Let us assume our slug has an MV of 950 FPS and it takes exactly 0.700 sec. to reach a target at 200 Yards. The exact velocity it will be travelling at 200 Yards will depend on its drag curve, but it will be between 777 and 783 FPS.
The four different drag models I am using for this article, and the corresponding BCs calculated using them, are as follows:
Note that the BC changes because of the drag model chosen, but in every case the slug is doing the same thing!
Remember, our slug is leaving the muzzle at 950 FPS, and arriving at 200 Yards 0.700 sec. later. However, what happens in between varies for the different drag models.
In particular, the drag between 800-1100 FPS is very different depending on the drag model chosen. Our typical slugs (as in SLG0) are not even close in shape to the G1, G7, or even the RA4 models, as you can see!
Until recently, there were not actually any drag models that closely resemble the flat-nosed airgun slugs we predominantly use today. That’s why virtually all BCs for slugs are calculated using the G1 drag profile.
Miles Morris, a UK Ballistician, calculated what the Drag Coefficient (Cd) was for typical airgun slugs, and that is now presented as the SLG0 drag model.
It is available in the “Easy BC” and “MERO” apps, free of charge, from George Conway https://gpc.fotosoft.co.uk/ where you can download versions (and other airgun apps) for most common operating systems.
How Does The Choice of Drag Model Affect the Optimum MV?
First, let’s look at the G1 Drag Model, which is the most common one used for slugs. One of the charts available in the MERO Ballistics Calculator is a Windage chart for “Wind Drift (Optimum)”.
The variables are Range, Wind Speed and Direction, and Ballistics Coefficient (and you must select the drag model desired).
For these charts, I chose a range of 200 Yards, a Crosswind of 10 MPH, and I used the BCs shown above for each different drag model. Note how different they are!
First is the chart showing the optimum MV (for minimum wind drift) for the G1 drag model, at a BC (G1) of 0.157:
As you can see, the least amount of wind drift at 200 Yards occurs at an MV of 850 FPS when using the G1 model. The next chart is for the G7 drag model, at a BC (G7) of 0.0764:
So, if your slug conforms to the drag profile of the G7 model, the optimum MV for minimum drift over 200 Yards would be 970 FPS.
The next chart is for the RA4 drag model, using a BC (RA4) of 0.144. The optimum MV for a .22LR bullet at 200 Yards is 1000 FPS.
Lastly, we have a chart using Miles’ new SLG0 drag model, using a BC (SLG0) of 0.151:
The SLG0 drag model indicates clearly that the optimum MV for our slugs should be about 990 FPS. This is a calculated drag model, and needs a lot of confirmation through testing, but it does agree so far with the data we have.
Certainly our slugs are a very poor fit to the G1 drag model, showing less drag than it would predict as the velocity increases, just like the SLG0 model predicts.
The result is that the optimum MV with slugs is likely closer to 1000 FPS than to the 850 FPS we find with pellets.
We really need to convince the slug manufacturers to stop using the G1 drag model and work together to develop one that more closely matches the slugs they sell!
The way things are, using the G1 model leads us to expect better downrange performance than we will see as the slug slows down, and also to reach incorrect conclusions about the optimum velocities we should be using.
All the G1 model does is enable the manufacturers to make claims of higher BCs!