Unfortunately, the two English ounce units don't agree: the avoirdupois ounce is 7000/16 = 437.5 grains while the troy ounce is 5760/12 = 480 grains. Conversion between troy and avoirdupois units is so awkward, no one wanted to do it. The troy system quickly became highly specialized, used only for precious metals and for pharmaceuticals, while the avoirdupois pound was used for everything else.
