20.5 greams is likely the bullet weight.
That is about 14.5gr. per gram, isn't it? Memory sucks with some conversions.
300gr. (297.25gr.) should work then.

My bro drove a 300gr. FP Hornady, I think, out of his Sharps at 2,260fps, through the rib cage of about a 500pound calf moose - long yearling - 1 1/2 year old bull. Made a 1" diameter exit hole in the hide.

Hell of a good bullet - absolutely sacked that moose.