13 grains would be just under the weight of 2 Water Gremlin Size BB shot (15 grains for 2/7.5 grains each).

If you use Dinsmore tin shot, then your 13 grains weighs just slightly more than one Size AAA (11.7 grains).

Your weight will be more evenly distributed with the miniheads though than with shot.

As for the weight comparison of grains to ounces, there are approximately 435 grains to the ounce, or approx 109 grains to the quarter ounce.

Mike