I am a shooter who competes in still-target shotgun (NWTF turkey shoot) events. We are trying to put the most possible shot in a 3-inch circle at 40 yards with our 12 and 20 gauge shotguns using factory turkey loads and turkey chokes. I have noticed that the weather plays an extremely important role in the ability of a gun/shell combo to produce tight clusters of shot on any given day. For instance, hot weather always seems to produce denser patterns than cold weather.
However, there are other weather factors that make a difference that I don’t understand as well. I’ve seen the quality of patterns change within a given day when the temperature remained relatively constant. I suspect that it must have something to do with changes in barometric pressure, air density, or some other factor. Can you elaborate on this and provide me any guidance on how I might predict good or bad times?
Thanks for your great question. It isn’t often that I get questions about atmospheric effects on shot patterns, but they can indeed be significant, just as they are in other sports, such as baseball or golf.
Air density exerts a drag force on any object moving through it. The higher the density, the more drag is exerted. This drag serves to slow down the object moving through it.
A smooth or nearly smooth object (such as a bullet or golf ball) will normally fly in a relatively straight path when subjected to this air drag.
However, as all golfers know, if the object has side, top, or back spin, its path will curve in response to the air drag force.
Now, lead pellets fired from a shotgun end up being far from smooth spheres. Those toward the rear of the load are subjected to setback forces when the load is fired, which deforms them and creates flat spots. Aerodynamic forces acting on these flat spots move the pellet transversely off track, as well as create spin, which does the same thing.
OK, this is a long-winded explanation of what happens when a non-spherical pellet moves through the air. The main point here is that these random aerodynamic forces open up (disperse) shot patterns. This effect increases with increasing air density. There is one other atmospheric effect that I will save until later.
So now to your question: what atmospheric factors affect shot dispersion? The answer is, anything that affects air density, which are pressure, temperature, and humidity. Of these, air temperature has the largest range and gives the largest changes in air density. The higher the temperature, the lower the density.
In absolute terms, air temperature changes from summer to winter can cause air density variations of up to 25%, with corresponding changes in shot pattern dispersion (tighter patterns when it’s hot, more open when it’s cold).
Within a given day, normal air temperature variations can cause air density, and thus shot pattern, variations of as much as 8%. Normal air pressure changes within a single day can change patterns as much as 3 or 4%.
Humidity effects amount to no more than 1 or 2%, even in the summer. Contrary to what most people think, the more humid the air, the lighter (less dense) it is.
One other atmospheric factor, atmospheric turbulence, can cause shot pattern changes from hour to hour within a given day. When wind blows over the ground, it creates random eddies of different sizes, some quite small, within which the air is moving in different directions. On a hot day, there are also thermal eddies caused by hot air near the ground rising. This combination of mechanical and thermal eddies makes up what is caused atmospheric turbulence.
When a pellet ensemble (or pattern) traveling through the air encounters atmospheric turbulence, different portions of the pattern can fly through different turbulent eddies of air moving in random directions. These can impart a small transverse movement to the various pellets, which has the overall effect of opening up the pattern.
Steve, I’m sure that’s more than you ever wanted to know, but that’s what happens when you ask a scientist a question!
Best wishes and happy shooting,