There are 437.5 grains per ounce (7000 divided by 16). Those "slugs" aren't slugs at all, they are 300-grain bullets. As such, they are much smaller in diameter than a 12-ga SLUG, (maybe .50 as opposeed to .70) and they have a much greater sectional density and probably a higherballistic coefficient as well. This means they hold velocity alot better than a slug, and will also out-penetrate any full-bore size slug, even one weighing a lot more. You are comparing apples to pumpkins here!

You just cannot evaluate a bullet using the same criteria you use to evaluate a Foster or Brenneke-type shotgun slug. When you use those loads, you are in effect turning your shotgun into a big-bore rifle like a .45/70. I am assuming that your shotgun has a rifled bore, as those "slugs" will have to have a spin to work.