Doing the conversion - 20 milligrams is 0.308647 grains So you're correct.

(had to use Google - I only know that there are 7000 grains to the pound)

I am GP and did more research and need to post a correction but it's too late to edit my former comment.

The grain, in terms of weight, is 64.8 milligrams, or much more than a even a course grain of sand, which it turns out, is somewhere around 1 milligram, ranging from 0.04 mg for fine sand to 4 mg for very course sand, average around 0.3.