The parameters of shotgun design are gauge, choke, barrel length, and shell. These parameters interact, so their ultimate effect on performance can not be isolated.
Gauge refers to the inside diameter of the barrel – the bore. Shotguns are normally smooth bore (not rifled). There are four common gauges, ranked from small to large: 28, 20, 16, 12, 10, where the number refers to the weight, in fractions of a pound, of a solid lead ball with the diameter of the bore. Thus a 16 gauge bore ideally passes a lead ball weighing 1 ounce. Of course, shotguns do not ordinarily fire such projectiles; it is a metric inherited from cannon design. The smallest shotgun bore size does use a caliber designation, the .410, which is a smaller than 28 gauge.
All other parameters held constant, a gross characterization of the practical effects of each next larger gauge are: the gun is 8 ounces heavier; the range is 5 additional yards; the shot string is shorter; a more powerful recoil (kick) is transmitted to the shooter’s shoulder. A 16 gauge shotgun has an effective range out to 50 yards. In the American market, the 16 gauge was mostly dropped from production in the 1970s, so that 12 and 20 gauge are by far the most available shotguns today.
Barrels come in various lengths, typically from 24″ to 34″. Effects of length on shot speed and pattern are functionally negligible. The functional distinctions are likely only experienced in gun handling – speed, balance, sighting. In such cases, try several lengths and pick the one or two that feel best in your typical shooting situations. Or, for an inherited gun, get used to its configuration. Unless one is a very talented shooter, any gun will likely perform better than its shooter.
Shotgun bores are not cylindrical. The chamber, in which the shell is fired, has a larger diameter than the bore, to accommodate the shell’s paper or plastic jacket. The forcing cone between the chamber and the barrel then restricts the shot string to the effective bore of the barrel. At the muzzle end of the barrel, there is optionally a further restriction called the choke, which affects shot concentration at the target. The shot pattern expands when it leaves the muzzle and at the target has a Gaussian distribution. Barrels without a muzzle restriction have no choke. Barrels with full choke have a 0.35 in. constriction. A commonly used choke is called ‘modified’ and has a 0.2 in. constriction.
The practical effects of different chokes relate to the percent of shot delivered to a 30 in. circle at 40 yards. It ranges from 40% to 60% to 70% for no choke, modified choke, and full choke respectively. Only a fraction of the shot is needed to intersect the target to achieve the desired effect, and there is more chance of that with less choke (shot concentration). Thus, those of us who are not fully competent with aiming will have more success with less choke. My 16-gauge M37 came off the shelf with 28″ barrel and no choke.
The standard shotshell is 2 3/4′ long with brass cap and paper or plastic hull. Longer ‘magnum’ shells are also available if the gun is designed for them. In the 1960s, there was an attempt to color code hulls by their gauge, but for 12 ga. its pretty much red or most other colors. 16 ga. seems often to appear in purple. 20 ga. is mostly yellow. As it is dangerous to mix gauges (a smaller ga. shell could jam the barrel and cause an explosion), and color coding is not an enforced standard, care in chambering is necessary when using unfamiliar shells.
The shot inside the shell is typically sized from 4-9. For lead shot, 4 is for turkeys and such, and 9 is for skeet. 7 1/2 is a typical all-purpose lead size, compatible with trap shooting, and will leave some of the bird for the table. Lead shot has offered the best bang for the buck by far, but its toxicity to wildlife has rendered it unusable in field and wetland for conscientious shooters. Further, it is not legal to use lead shot in the US in wetlands.
Steel shot is less effective, and some think it may cause more wear on the inside of the gun barrel. Buying steel shotshells with a buffer added to the shot may help maximize performance and minimize wear. Other choices are bismuth alloy and tungsten matrix (Hevi-Shot). Bismuth closes the gap between weight of steel and lead shot, while tungsten alloys weigh as much or more than lead.
Rules of thumb for using non-toxic shot (compared to #6 lead shot):
- steel #4 (down 2 sizes)
- bismuth #5 (down 1 size)
- tungsten #7 or #7 1/2 (up 1 size)
These shot materials are all harder than lead shot and may not be compatible with full choke tubes.