.040" constriction in a standard 12 gauge gives about 11% bore reduction. The more modern .030" constriction for full choke with modern shells gives about an 8% reduction. You can figure these percentages for any size you desire, just remember they are figured from the diameter Sq'd thus for a 20 gauge using modern 8% constriction √(.615˛ x 92%)=.590" or .025" constriction.

Incidentally for the original poster a 13 gauge bore is undersized for a 12 an 11 gauge is overbore.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra