Someone correct me if im wrong here.

The chance of the drop being my main is 1/13.
The chance that it will drop to me is 1/72 (its really the number of participants, but from what im hearing this is no low manning affair.)
making the chance that I get the drop for my main 1/936
and that's assuming the fight occurs every time and the weapon drops every time. which may not be true.

if the fight occurs one chance in 5 as someone said on the forum, that could make this 1/4680. again not accounting for drop rate.