Shotguns: The Flight of the Pellets

photo from ar15.com. When a shot pattern leaves a shotgun barrel the choke of the barrel, the wad, the size and density of shot, and even the density of the air all affect how it will pattern downrange. Understanding the physics of a shotgun pattern will help you choose the right pellet size and choke for the situation.

If you were to fire a shotgun in the vacuum of space, the pellets would stay nestled in their shot cup forever as they flew through the universe. However, on Earth, atmospheric pressure affects your shot load as soon as it leaves the muzzle on its way to a clay target, a turkey, or a bird in flight.

Hot propellant gasses expand suddenly as they exit the muzzle, creating the shock wave that we hear as muzzle blast. In the first three feet of travel, air resistance slows the load about 100 feet per second (fps). The faster you drive pellets through the air, the faster they lose velocity. Increasing velocity by using fast loads does increase energy, but only somewhat. The best way to hit birds harder is to shoot larger, heavier shot.

Air resistance pries open the petals of the shot cup, releasing the pellets. Shot cups protect barrels from hard pellets, such as HEVI-Shot or those made from steel or tungsten-iron. They also prevent soft-lead pellets from being deformed in the barrel. A spent cup shows dimples caused by the inertia or “setback” forces that drive the pellets back into the plastic as it accelerates.

Some loads contain a ground-plastic buffer that protects lead pellets at the rear of the shot cup from being crushed by the weight of those at the front, enabling them to retain their round shape and fly truer.

This is an illustration of Federal Premium's Black Cloud shot coming out of a shot cup after the payload and shot cup have left the shotgun barrel. Black Cloud uses Federal's Flitestopper pellets and the Flitecontrol wad to put more hits on target downrange.

Just five feet from the muzzle, the load will already have lost about 150 fps in velocity. As it encounters air resistance, the pattern begins to open up, with pellets starting to veer off in different directions. The pellets at the back of the pattern are “drafting,” like racing cars tucked in behind the leader of the pack. These trailing pellets will eventually form the central core of the pattern—the dense part with which you hope to hit the target.

The tighter the choke you use, the longer and narrower the shot cluster will be as it leaves the barrel, and the more “drafting” pellets that will remain in the pattern core. The load will also pattern more tightly in thin air that in heavier air due to the lower air resistance. Air is not uniformly dense—it’s denser at sea level than at high altitude, and cold air is denser than hot air. The difference may not be much, but it’s no coincidence that the record shot pattern (most pellets in a 3-inch circle) was shot in the thin air of a hot, dry day. Hedge your bets in the cold, dense air of late-season by shooting high-velocity loads or by choosing one size larger shot than you ordinarily would.