Sommerset throws his gun...
...After seeing Tracy head in the box. Curious, maybe he Also felt tempted to shoot John Doe.
shareIn the '94 version of the script, Sommerset points his gun at Mills to dissuade him from killing John Doe. Mills points back at Sommerset and after an exchange of dialogue, that's when Sommerset throws his gun and begins to dissuade Mills through words alone.
They changed this part in the final version, which leads me to believe that Sommerset immediately threw his gun so that Mills would remain calm and see that he would not be a threat.
It's hard not to get nervous when a person with a gun wants to convince you of something.