100 yards. Can't believe I left out that important detail.

More information:

12.8 grains: unknown velocity (chronograph error, or rather user error), ~2" group
12.9 grains: 1,773 fps average, 22 fps s.d., 2.918" group
13.0 grains: 1,780 fps average, 21 fps s.d., .966" group sans flier
13.1 grains: 1,794 fps average, 35 fps s.d., .908" group sans flier
13.2 grains: 1,801 fps average, 28 fps s.d., 1.115" group

Looks like 13.1 grains is the winner, especially since if I'm off slightly when measuring the powder charge, it'll still group decently.

Formerly TN Lone Wolf

"We are only as strong as we are united, as weak as we are divided." - J.K. Rowling