Daryl,
You are correct. Fiber length and basis weight of the paper. Official targets are actually printed on a low grade, short fiber paper stock and are a heavier weight than standard 20 lb. copy/printer. The weight of the paper actually refers to the weight of 1000 sheets of the paper, as I recall. Card stock is on the order of 50 lb. weight. Not all printers are "happy" with 50+ weight papers/card stock.
J.B.