Above, a Kent  1-1/4 oz. Bismuth No. 4 shotshell. Am unbuffered load, the shot is erratic in size and shape, and has the unfortunate tendency to fracture in the barrel, embedding itself into the wad . . . and never leaving it as a result. 

When it comes to wounding ballistics, the most important things I've learned came directly from the late, great Dr. Martin Fackler. Shotshell ballistics are a mess, particularly due to chef's mix of shot types: now, hardly just lead and steel (soft iron), but bismuth, various tungsten alloys of different final densities, along with some interesting but strange duplex loads. So, what's the problem?

Above: a rare look at decoying pigs. When this becomes commonplace, shotshell gel penetration numbers will be more accurate.

PENETRATION

It is universally agreed that penetration and the permanent cavity are the most reliable barometers of lethality. However, the 10% ballistic gelatin refined by Dr. Fackler and others as a tissue simulant has very little to do with anything that flies. From its inception, ballistic gel and the penetration numbers derived from it is severely flawed with shotshell pellets. The reason is simple: ballistic gel was developed to simulate human muscle, not bird tissue, and the development stems from shooting pigs. Swine muscle approximates human tissue.

Ballistic gel does not attempt to model skin, bone, feathers, down, or gizzards. As there is no accurate model for bird penetration, the gel tests can only be truly useful when pigs can fly.

ROUNDNESS

The excellent programs by Ed Lowry and now by KPY Ballistics https://kpyshotshellballistics.com/ have to assume that pellets are perfectly spherical. They also have to assume that a shot # actually means an exact diameter, which it clearly does not. Perfectly round is the best form factor for downrange ballistics, and perfectly round is not what we get with lead, bismuth, and several other gravel level shot materials. In one example, Winchester Blindside, the pellets are intentionally not round and have no spherical diameter. The size of a perfectly smooth, spherical object can be accurately defined by a single dimension, which cannot be done for anything that is not perfectly round and smooth. Yet, a single dimension is what shot ballistics is based upon. Downrange strike velocities, pig penetration, and so forth are all based on the spherical form factor. It is a version of garbage in, garbage out.

BONES

Most birds have hollow (pneumatic) bones with air sacs to aid respiration. With air-filled bones that connect to their lungs and air sacs, birds are able to increase their oxygen intake capacity. There is no accurate hollow-bone modeling in shotshell wounding ballistics that I am aware of.

DENSITY

Density is the most important factor in shot pellet potential. Density is the reason that steel and bismuth can never equal lead. Steel does have the advantage in sphericity and resistance to deformation. Bismuth does have better density than steel, but can be severely handicapped by lack of roundness, lack of consistent sizing, brittleness and resultant pellet fragmentation. The primary saving grace of bismuth is that bismuth is “lead only” shotgun barrel and choke friendly.

FINALLY

We know that lead has the longest track record, followed by soft iron commonly referred to as steel. We also know that steel isn't going to get any rounder or denser. We know, at the very least, that bismuth is density-limited. The performance-minded wingshooter is left with only a few good options: use lead when possible, or go with either a tungsten alloy, or a duplex steel / tungsten load knowing full well that the additional lethality and additional range comes solely from the tungsten alloy pellets.