There are, by my count, 4,337 good reasons to shoot light loads in a shotgun, but let's start with the top six.
One, they are more comfortable. It matters not whether it is 1 ounce in a 12 gauge or 3/4 ounce in a 20, light loads are more pleasant on the shoulder and the ears.
Two, light loads promote better shooting. Because they are easier on you, they are less flinch-inducing. So, you miss fewer targets because you cringed involuntarily at the instant of firing. More fun to shoot means more practice, which also means better shooting.
Three, they are easier on your gun. Not a big concern if you don't care about the gun, but a very big one if you invested in a good gun that you hope will last forever.
Four, the ammunition is lighter to carry. Not important? Remember that after a couple of hours in the field, battling high grass and briars.
Five, light loads are cheaper. If you are loading them yourself, you use less powder and shot. Buying them, you generally find good light loads (in the form of trap or skeet loads) for less money than the red-hot big-bangers the stores sell for hunting.
And six--I'll get to six in a moment.
What is a light load?
In a 2¾-inch 12 gauge, generally anything up to 1…› ounces, at a velocity up to 1,150 fps, qualifies as light--and I like them considerably lighter than that. There are light loads in the other gauges, and the same principles apply.
The sixth reason, of which I am convinced but which is difficult to quantify since absolute proof is hard to come by, is that light loads put a superior pattern into the air--regardless of gauge--and this pays off with more hits and cleaner kills.
The problem is that a shot pattern in the air is an ever-changing ephemera that does not exist at the muzzle; from there, it develops, flows, changes, spreads, and eventually dies; by the time the last shot pellet hits the ground, the pattern has long-since disappeared and become just random, scattered pellets. No one has filmed an actual shot pattern, and all the pattern boards and sheets of paper do nothing except give you a rough idea of pattern spread and shot concentration--and then, only on a flat plane at one moment in time.
How long was the pattern? No idea. Was it a ball? An oblong? A cone? If a cone, did the point lead, or was it the base?
According to pattern theory, the shot column in the shell should be approximately as long as it is wide. One ounce of shot in a 16-gauge muzzleloader is, according to most experts, the ideal starting point. The 16 gauge is the same diameter as a lead sphere 1/16th of a pound, or exactly 1 ounce. Divided into pellets, the column should be about the same length as it is wide. In theory. In a muzzleloader.
With modern shot cups, this ideal is found, more or less, in a 1-ounce 12-gauge load.
Light loads are comfortable to shoot in easily carried lightweight shotguns — an advantage savvy upland game hunters consider.
As the shot leaves the muzzle, it should spread evenly and consistently. At optimum distance--25 to 40 yards--it should expand into a pattern that resembles a large ball. Tests have shown that the best patterns (measured in the conventional manner) are obtained when velocities are moderate--1,050 to 1,150 fps at the muzzle. Anything more tends to scatter the shot unevenly and the patterns become ragged.
Anyone looking for things to criticize in this argument will, by now, be gnashing his teeth and muttering, "What about choke? What about buffers? What about shot deformation?"
There are so many variables in shotgunning that it is difficult to lay down one hard-and-fast rule. I am presenting this as a general theory of what tends to work and why. In shotgunning, the key seems to be moderation in all things--shot charge, velocity, and choke being the main factors to consider.
Does it work in the field?
In 2008 I hunted wild pheasants on a friend's farm in South Dakota. A half-dozen of us were hunting with English shotguns more than a century old. All had 2½-inch chambers. My gun, an E.M. Reilly boxlock (shown in the opening photograph), has 30-inch Damascus barrels, and I was shooting Baschieri & Pellagri 30-gram (1 ounce) 12-bore ammunition. Very light, very gentle.
My gun has one wide open and one tightly choked barrel--a typical configuration for a gun from the 1890s. It also weighs just 6¼ pounds. After a somewhat ragged start and a little ammunition switching the first day, I settled down for the next four days and managed to down 13 pheasants with 15 shots. That's one over the limit because one bird was not recovered. All but one were taken with the Cylinder barrel (and, interestingly, the lost bird was shot with the tight barrel). We were shooting in high grass over dogs, with pheasants flushing every which way. This was by far the best shooting I have ever recorded on any wild birds. I was very careful to identify roosters and pick my shots, and I did not try any at long range; probably my longest shot was 40 yards.
And, as I walked back late in the afternoon with that lovely, light gun, I was still carrying it in a hunting position, ready to shoot.
Which is a seventh good reason to shoot light loads: You can get a gun at a weight you like and the balance you seek without worrying about the effect of heavy loads on either you or your gun. You can carry a lighter gun in the classic "ready to mount" position longer, as you struggle through the brush, and be ready for those unexpected flushes that always outnumber the expected ones by about 10 to one.