Sources and links, dear....or you get no hard candy or Freedent.
That was discussed in the PBS Frontline
two-part special on Adam Lanza, and on CNN
. Four of his "spent" magazines were found to contain 10, 11, 13, and 14 rounds remaining (plus one in the chamber in the last one that held 14-rounds). He was swapping when they were 1/3 to 1/2 full, self-limiting his magazine capacity.