1/65 gr = 0.002285 oz 1 mg = 1/65 grain 1/65 gr = 0.002285 oz does this help? if 1/65 grain = 1mg, and 1/65 grain = .00285 ounces then does 1 mg = .002285 ounces?
For Avoirdupois ounce (the regular ounce), there are 28.35 grams, so 28,350 milligrams. For a Troy ounce (like how gold is measured), there are 31.1 grams, so 31,100 mg.