Velocity and Wind Drift

[rsterne]

I decided to do this thread because of some misconceptions I keep running into regarding wind drift with airguns.... The comments inevitably blame the wind drift that plagues us on the low velocities, and consequent long flight times to the target.... In fact, airguns often operate in a "sweet spot" in terms of low wind drift.... Think I'm crazy?.... Consider this....

The amount of drift is not proportional to the flight time, but rather to the DIFFERENCE in flight time between the real world and what would happen to the same pellet/bullet starting from the same velocity in a vacuum.... The higher the drag, for a given Sectional Density, the quicker the projectile slows down, so the greater the difference between its flight time in air and in a vacuum.... The problem is, that the drag increases many fold as the projectile breaks the "Sound Barrier".... There are several Drag Models, which represent various shapes, and here are a few "drag curves" showing that rapid increase in drag in the Transonic Region (Mach 0.8-1.2)....



If we use a typical drag curve, represented by the G1 Model (the orange line above).... and then use various Ballistics Coefficients, we can plot the wind drift for various muzzle velocities over any range.... I chose 200 yards because that is what is used at the Extreme Benchrest event, just run in Arizona.... Here is what happens for BC's of 0.05, 0.10, 0.20, and 0.40.... which spans pretty much anything we might see in airguns.... These charts are for a 10 mph crosswind, calculated using the JBM Ballistics Calculator.... http://www.jbmballistics.com/cgi-bin/jbmtraj_drift-5.1.cgi



The first thing you will notice is how important the BC is for reducing wind drift.... The biggest influence on BC is the Sectional Density of the bullet, with the shape of secondary importance.... Pellets or roundball might behave roughly like the yellow curve, a chunky slug like a 100 gr. in .357 cal might be like the blue curve, a long thin bullet like a 90 gr. .257 would be something like the orange curve, and a very well designed boattail spitzer might be like the grey line.... The other thing that is apparent is that the drift does NOT get less as the velocity increases above what we usually run with airguns.... In fact, it gets WORSE as we push Supersonic, and you have to reach velocities unheard of with airguns to get back down to the same amount of wind drift we get.... Counterintuitive maybe, but FACT.... Let's concentrate on bullets we might use at the BigBore shoot at the EBR....



The important thing here is to look at the velocity where the wind drift is at a minimum.... For any BC we might use at that event, shot at 200 yards, the least wind drift occurs when using a muzzle velocity of about 900 fps.... True, the curves are pretty flat either side of that (particularly with a high BC).... but you have to push the bullets more than twice that velocity before the drift once again drops to what we achieve with the velocities we already use.... So, when you are cursing the wind when shooting an airgun.... don't blame it on the low velocities we use.... You are looking in the wrong place if you do.... Instead, you need to be looking at a bullet with a better BC.... When shooting around that 900 fps velocity, if you double the BC, you will cut the wind drift roughly in half....

The actual MV you choose will be governed mostly by where your bullet shoots the most accurately.... Anything between 800-1100 fps makes sense, but the closer you stay to 900, the less drift you will have to deal with.... Yes, the trajectory won't be as flat as if you push the bullet at 1050 fps, but gravity is a constant and can be allowed for by zeroing your scope.... The wind is anything BUT constant, so IMO you need all the help you can get....

Bob

[rkr]

Shouldn't we consider the velocity at the target as well? The bc should be at it's lowest in between the muzzle velocity and velocity at the target to minimize the drift over distance.

[MJP]

For me personally I can live with wind drift for flatter trajectory any day. Don't much care for mortar like trajectory on any of my guns.
Marko

[rsterne]

rkr.... The calculations take into account the decay in velocity as the projectile moves downrange.... That is why the optimum muzzle velocity is higher when shooting at 200 yards if the BC is lower, because the bullet slows down more quickly.... The key is where along the drag profile the bullet is at any given moment, and that is all worked into the data presented.... Of course if the drag profile of your bullet doesn't match that of the G1 projectile, then there will be a slight variation, but unless you have the complete drag data for every velocity from muzzle to target (eg. using a LabRadar), the best we can do is use the closest existing drag model.... The commonly used one is the G1 Drag Profile, shown below in more detail for the velocities that interest us.... If you notice, the drag is at a minimum at about 600 fps, and yet the optimum MV over 200 yards is about 900.... exactly because the bullet slows as it goes downrange, and we want it to spend as much time as possible drifting sideways the least amount....



Marko.... The decision to opt for flatter trajectory over lower wind drift is completely understandable, particularly if you are shooting a bullet with a high BC anyway, where the drift is relative low.... For example, if you have a BC of 0.40, using a MV of 1100 instead of 900 only increases the drift at 200 yards by 1.2", but if you are sighted at 100 yards, and are presented with a 200 yard shot, the bullet starting at 900 fps drops an additional 45" at 200, while the same bullet starting at 1100 only drops 31"....

However, if you are shooting at a known range, like at the EBR, you can sight your gun in for that distance, so trajectory doesn't matter at all.... In that case, the 1.2" less drift could mean the difference between a 10 and a 9.... With a much more typical BC of 0.2, a bullet starting at 900 fps has 2.1" less drift than one starting at 1100 fps.... which could be the difference between a 10 and an 8....

For hunting, or any situation where you are shooting at unknown ranges, your point is well taken.... My typical Varmint hunting is done anywhere between 30-110 yards, so although the least wind drift occurs at around 900 fps, I set up my guns for about 960.... The additional drift is small, and far outweighed by the flatter trajectory.... when I don't have the luxury of sighting in at a known range.... However, accuracy trumps everything else....

Bob