The Physics of Shotgunning

How Einstein and Newton can help you squeeze the most performance out of your scattergun

Recoil can be explained by Newton’s Third Law of Motion: For every action, there is an equal and opposite reaction.

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Recoil can be explained by Newton’s Third Law of Motion: For every action, there is an equal and opposite reaction.

“Nothing happens until something moves.” —Albert Einstein

Shotguns, of course, are bound by the laws of physics. Einstein would probably say that when your trigger finger moves, it sets in motion a series of events that can be predicted by basic scientific principles. For waterfowl hunters, understanding the interplay between these principles and your gun’s performance in the field can help you choose the most effective guns, loads, and chokes for your type of hunting.

Recoil

“Conservation of momentum” is an important component of Isaac Newton’s Third Law of Motion, which should be apparent to anyone who fires a 3 1/2-inch shell from a light shotgun. Guns recoil because for every action, there is an equal and opposite reaction.

Recoil is a function of gun weight, ejecta (the weight of powder, wad, and shot), and ejecta velocity. As a general rule, if you change gun weight by 10 percent, you change recoil by 10 percent. Changing velocity or payload weight by 10 percent results in a 20 percent change in recoil. Using 1/8 ounce more or less shot, or using a shell that’s 100 fps slower or faster, will also cause a noticeable change in recoil. A slower, lighter load is the best recoil reducer of all.

Recoil and felt recoil, or “kick,” are two different things, although they’re related. Two guns can generate the same amount of recoil, but the one that fits the shooter better and has a softer recoil pad will kick less. Gas semiautos don’t have less recoil than other shotguns, but they kick less because the piston, bolt, and other moving parts absorb some of the recoil energy for a split second before releasing it. Because the recoil is “spread out” over time, it feels like a longer, softer shove rather than a sharp kick.

Air Resistance and Velocity

As the payload exits the muzzle, the frictional force of air resistance opens the petals of the shot cup, slowing it down and releasing the pellets. Air resistance slows the pellets too, and the pattern starts to open.

High-velocity loads hit harder than lower-velocity loads at short and mid-ranges, but the difference narrows as distances increase. That’s because the faster you drive pellets against air resistance, the faster they slow down (Newton’s Third Law again). As an example, take two loads of steel 2s, one of which is a 1,500 fps load and the other a 1,275 fps load. Despite the initial 225 fps difference in velocity, by the time they reach 50 yards, the 1,500 fps load is traveling at 649 fps and the 1,275 fps load is moving at 594 fps, just a 55 fps difference.

Choosing bigger shot is one way to gain velocity, because pellets with more mass have greater inertia and retain velocity better than smaller pellets do. If we compare two 1,500 fps loads, one with 2s and one with BBs, the BBs will be traveling 723 fps at 50 yards, while the 2s will have slowed to 649 fps, even though they started at the same velocity. Choosing bigger pellets in loads of the same weight increases velocity and energy without adding recoil, although that advantage comes at the expense of pellet count.

Air resistance often has the effect of opening patterns more quickly, so, in general, slower loads pattern more tightly than faster ones. Another variable is air density, which changes with temperature (cold air is denser than warm air), with humidity (dry air is denser than humid air), and with elevation (air at sea level is denser than air in the mountains). Denser air means greater resistance, so a gun, load, and choke combination that shoots a 60 percent pattern at sea level may shoot a 70 percent pattern at 5,000 feet.

Kinetic Energy 

Kinetic energy is useful to waterfowl hunters as a predictor of penetration. It takes about three to four foot-pounds of energy for a pellet to adequately penetrate a duck, while five to seven foot-pounds is a good range for big geese.

You can increase kinetic energy by shooting faster shells. You can also boost energy by shooting pellets with greater mass, by selecting either a larger pellet or a denser shot material such as bismuth or tungsten-iron. Denser nontoxic pellets retain velocity and energy better than steel pellets because they are smaller than steel pellets of equivalent mass. If two pellets have equal kinetic energy, the smaller pellet will penetrate more deeply.