The key to the superior performance of Super-18 shot is the density. At 18g/cc (about 60% denser than lead), the penetration energy is so high that it enables one to go down significantly in pellet size, to greatly increase the pellet count and pattern density, while simultaneously increasing the penetration depth of the pellets into the target. This enables one to greatly increase the performance of a typical big bore shotgun, or to go down in bore size or payload without giving up performance - or both.
What makes TSS so effective
When it comes to shotgun pellets and fowl, what determines a pellet's lethality is penetration. And penetration depth is determined by the energy per surface area of the pellet. Weight does not determine this - it's weight per area. If two pellets weigh the same, but one is smaller, the smaller one will penetrate deeper because the energy is focused on a smaller surface area on contact and is not dissipated over as wide an area.
As for comparing 18g/cc tungsten pellets vs lead (11g/cc), a Super-18 pellet will have about the same penetration energy as a lead pellet 5 sizes larger. So, a Super-18 #9 pellet will penetrate into soft matter at approximately the same depth as a Lead #4 pellet. However, in real life scenarios, on real birds, the Super-18 will actually do better vs lead, because of hardness. It will break bones better, or any other hard material, because of it's hardness and lack of flattening out when it comes into contact with any surface.
For illustration, here's a comparison of pellets of various shot material. Let's say that each pellet is going 1100 fps (muzzle velocity), at sea level and at 70 deg F. Here's the distance each type pellet will give you 1.25" of ballistics gel penetration, as well as showing the number of pellets per ounce. When you think about the penetration per pellet along with the number of pellets available to penetrate the target, these numbers will illustrate why density makes such a huge difference in a shotshell's performance on fowl.
8g/cc Steel #2 (123/oz) – 45.7 yds
11g/cc Nickel Plated lead #5 (174/oz) – 60.4 yds
12g/cc Hevi-13 #6 (208/oz) – 60.4 yds
15g/cc Fed HWT #7 (220/oz) – 81 yds
18g/cc TSS #8-1/2 (300/oz) – 84.5 yds
In addition to penetration energy and pellet count, there is a 3rd factor that doesn't show up in the math - patterning characteristics. In a nutshell, the denser and harder the shot material, the better the pellets will hold together in a pattern.
As a general rule, in terms of penetration energy, Super-18 shot is equivalent to lead shot 5 sizes larger, Hevi-shot 4 sizes larger, and Heavyweight-15 1-1/2 sizes larger. So, Super-18 9s are equivalent to lead 4s and Hevishot 5s. But you get about 360 Super-18 9s per ounce, 160 Hevishot 5s to an ounce and 135 lead 4s to an ounce. Going up in density to 18g/cc Super-18 shot allows you to go down in size, thereby greatly increasing your pellet count and pattern density. It's a recipe for huge advances in turkey shell performance.
A question that always comes up with TSS, is what shot size to use.
Given that the whole reason TSS is so much better is due to the high density of the pellet material - which enables one to go down in shot size in order to increase pellet count and pattern density, without losing any individual pellet penetration energy or to actually increase that penetration energy - the question of optimal pellet size of how far down in size to go, is a good one to consider.
The good news is, with the available ballistics software, it's easy to come up with what the optimum pellet size for a given situation is. Here's how I look at it - using turkeys as the game for illustration purposes.
You have basically two things to think about - pellet penetration and pattern density. You need both to kill turkeys consistently and cleanly. And there is a trade-off of those two things with each shot size and payload. For pattern density, I have a minimum standard of 100 in a 10" circle. For pellet penetration - 1.25" in ballistics gelatin. (These standards will be obviously different for other game birds and waterfowl.) Both these minimum standards must be met in order to know you will have a dead turkey 100% of the time you put the core of the pattern on him, at whatever range within those limits. The ideal shot size is the one where you max out on both those minimums at about the same time. Then you aren't dealing in overkill on one, at the expense of the other - since a shortage of either will hurt you. That's why I generally prefer smaller shot sizes in the smaller bores, and larger in the larger bores - to match up the pattern density and pellet penetration - thereby maximizing the efficiency of your rig and ammo.
For illustration, let's say I have a 1-5/8 oz 20 ga load that goes 1150 fps MV. What's the optimum pellet size?
With number 8s, I know I can get 220 in a 10" circle at 40 yds. Using the .7 rule, that means I can get 100 in a 10" circle out to a little over 60 yds. And the 8s give 1.25" penetration out to 92 yds. So, I have a gap between the pellet penetration and the pattern viability of about 30 yds.
With number 8-1/2s, I know I can get 270 in a 10" circle at 40 yds. Using the .7 rule, that means I can get 100 in a 10" circle out to almost 70 yds. And the 8-1/2s give 1.25" penetration out to 82 yds. So, I have a gap between the pellet penetration and the pattern viability of about 15 yds. We're getting closer to the optimal size for the 20 ga.
With number 9s, I can get 310 in a 10" at 40 yds, so the numbers are 70+ for the pattern viability, and 72 yds for the pellet penetration energy. Those match up almost perfectly, with no gap between the pattern viability and pellet penetration. So, that's why I choose #9s for the 20 ga, for turkeys.
You can do that same exercise with any pellet size, payload, and game, to figure out the optimal pellet size for what you're trying to do.