Ammunition selection has a great deal to do with 'effective range' for a 12-gauge.
Heavy-Shot will add to it considerably, I'm sure.
Now.....there are those who will relate that '...in VietNam they had flechettes which would make one-shot kills at 100 meters...' Personally, I'd take that with just a tiny grain of salt. I've experimented with flechettes and found the 'effective range' to be more like 10 yards not 100. It's ultra-fancy design is no match for it's ultra-light weight and you're shooting Very ineffective bird shot with a spectacularly widening pattern (like 6 foot in diameter at 15 yards
)
If it worked, our government which can spend money faster than Niagara Falls can spend water, would use it. They don't. Stick with large diameter, heavy projectiles and you'll benefit from it.